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1 

2 

3 

1 

2 

3 

4 

5 

6 

< 


■r. 


1 


{COLUMBIA    VNIVERSITY  BIOLOGICAL    SERIES.    II.) 


Amphioxus  and  the  Ancestry 
OF  THE  Vertebrates 


y. 

r. 
■Si 

^  \ 


BY 


ARTHUR  WILLEY,    B.Sc. 

TfTOR   IN    lilOLOGY,    COLU.MIUA   COLI.EGE;    HaLFOLU   StI'DENT   OF   THE 

University  of  Cambridge 


< 

< 


■r. 


WITH  A   PREFACE 

BV 

HENRY   l-An<FIELD    OSBORN 


MACMILLAN    AND    CO. 

AND    LONDON 
1894 

All  rights  reserved 


LIBRARY 

NATIONAL  MUSEUM 

OF  CANADA 


St- 


A 


"r* 


\%  II 


•■    COPVRIGHT,    1894, 

Bv  iMACMILLAX  ANIJ  CO. 


Xorinooti  ^ttw. 

J.  S.  Cushingr  &  Co.  -  Bsrwick  &  Smith. 

Boston,  Mass.,  U.S.A. 


n 


Dcliicntcti 

IN'   CRATITUDE   AND   ESTEEM 

TO 

PROFESSOR   E.    RAY    LANKESTER,   F.R.S. 

BY 

HIS    FORMER    PUPIL 

THE   AUTHOR 


^^1 


..^^ 


T 

ture; 

ipc 
^tud 
Cxte 
ance 
ture 
|he: 
inert 
wide 

P] 
the 
thcii 
cenc 
men 
deei 
Lan 
chili: 
func 


PREFACE. 


This  volume  originated  in  a  course  of  University  lec- 
tures prepared  at  my  suggestion  by  the  author.  It  seemed 
important  that  he  should  bring  within  the  reach  of 
Students  and  of  specialists  among  other  groups,  his  own 
extensive  observations  upon  Amphioxus  and  other  remote 
ancestors  of  the  Vertebrates,  as  well  as  the  general  litera- 
ture upon  this  group.  While  our  detailed  knowledge  of 
|he  structure  and  habits  of  these  animals  has  been  rapidly 
increasing  in  recent  years,  it  is  still  in  the  main  very 
widely  scattered  in  monographs  and  special  papers. 

Probably  no  single  group  illustrates  more  beautifully 
|he  principles  of  transform  ism  ;  for  the  Protochordates  in 
their  embryonic  development  exhibit  remarkable  reminis- 
cences of  past  adaptations,  and,  in  their  adult  develop- 
ment, the  most  varied  present  adaptations  to  pelagic, 
deep-sea,  littoral,  free-swimming,  and  sessile  life.  As 
Lankester  has  shown,  the  Ascidians  alone  give  u.'^  a  whole 
chiipter  in  Darwinism.  But  degeneration  and  change  of 
function    constitute    only   one    side   of  their   history.     In 


vn 


Vlll 


J'KKFACi:. 


progressive  development  some  of  these  types  have  comi- 
to  so  closely  resemble,  superficially,  certain  of  the  larger 
groups  of  Invertebrates,  such  as  the  Molluscs  and  Worms, 
that  it  is  only  at  a  comparatively  recent  date  they  have 
found  their  way  out  of  these  groups  into  the  Trotochor 
tlata.  Many  of  these  misleading  resemblances  are  now 
interpreted  as  parallels  of  structure  springing  from  i)arallel.s 
in  life  habit,  seen  not  only  in  the  general  body  form,  biii 
in  special  organs,  such  as  the  breathing  apparatus  of  tiic 
Ascidians  and  Molluscs. 

By  the  side  of  parallelisms  are  real  invertebrate  and 
vertebrate  affinities ;  so  that  the  problem  of  resolving; 
these  various  cases  of  original  and  acquired  likeness  in 
their  bearing  upon  descent  has  become  one  of  the  most 
fascinating  which  modern  Zoology  affords.  For  example, 
among  the  real  invertebrate  ties  of  the  Protochordates  arc 
the  ciliated  embryos  of  Balanoglossus  and  Amphioxus, 
the  Tornaria  larva  and  ciliated  ectoderm  of  Balanoglossus 
The  nervous  system  of  Balanoglossus  presents  both  ver- 
tebrate and  invertebrate  characters  ;  the  respiratory  sys- 
tem is  identical  with  that  of  Amphioxus,  while  in  thi 
embryonic  development  there  are  many  resemblances  /;//</ 
sc.  In  short,  in  Balanoglossus  and  the  Ascidians  the 
invertebrate  type  of  structure,  whether  original  or  ac 
quired,  predominates.  But  in  Amphioxus  the  balance  is 
far  on  the  other  or  vertebrate  side  of  the  scale,  and  this, 
with  its  resemblances  to  lowvjr  forms,  gives  us  the  coi^- 


PKEFACE. 


\ 

'  have  come  , 

r 

i  the  larger  ' 
and  Worms, 
e  they  have 
;  Protochor- 
:es  are  now 
om  jxi  rail  els 
ly  form,  but 
rat us  of  the 

tebrate  and 
f  rcsolvin<r 
likeness  in 
)f  the  most 
or  exam  pi  f, 
lordates  arc 
Amphioxus, 
anoglossus. 
s  both  vcr- 
iratory  sys- 
hile  in  the 
lances  tutir 
cidians  the 
inal  or  ac 
balance  is 
e,  and  this, 
IS  the  ccn- 


ix 


necting  link  between  Protochordate  and  Chordate  orjran- 
isation.  Hefore  entering  into  any  of  these  discussions, 
the  author  has  given  :,  Miorough  systematic  and  structural 
treatment,  especially  of  Amphioxus. 

This  exquisite  form,  Amphioxus,  is  of  almost  world-wide 
distribution  and  has  enjoyed  the  attention  of  every  great 
zoologist  for  over  half  a  century,  yet  the  most  recent 
studies  upon  it  have  been  among  the  most  productive  of 
discovery.  Its  interest  and  value  as  an  object  of  biologi- 
cal education  has  steadily  increased  with  the  knowledge 
that  in  contrast  with  all  the  related  forms,  it  stands  as 
a  persistent  specialised  but  not  degenerate  type,  perhaps 
not  far  from  the  true  ancestral  line  of  the  Vertebrates. 

n.  F.  o. 


'T*^ 


m 


rx' 


i 


11. 


CONTENTS. 


»o« 

I'AC.I! 

INTRODUCTION i 

■I.    ANATOMY  OF  AMrillOXUS f 

IIlsHlKlCAl 7 

^:          IlAltll'S  AND    I)|>rKllUri(iN 9 

M           KXTKKNAl.    I'OKM 12 

'1&:                Cranium  and  Sensc-ur^'aiis l^ 

;fe?           InIKKNAI,    AnAIi'MV 22 

t                Atrial  Cavity 22 

Viscera 24 

CiiLiin 26 

J                 StruL-uin:  nf  l'iiaryi)\ 27 

Evolution  I  if  tlic  riumus  Cilaiul 29 

l'".nili)styU' 3' 

Branchial  liars 32 

Musculature 34 

NiiTi.s 38 

II.   ANATOMY  OF  AMrumxrs 46 

Intkknai.  AnaT'IMV  {■ontliinrJ) 46 

Vascular  System 4^ 

The  Excretory  Sys^teni 55 

Development  of  the  Atrial  Cavity 75 

Comparison  between  the  E\creti)ry  System  of  Amphioxus  and 

that  of  the  Annelids 78 

Nervous  System 82 

NoiKs •     ■     .•          .     -     , 98 

xi 


Xll 


cox  TEXTS. 


III.   DEVELOPMENT  OF  AMl'IIIOXUS 
Embryonic  Dkvkloi-ment 

Fertilisation  rami  Segmentation  of  the  Ovum 

Castrulation 

Growth  of  Free-swimming  Embryo  . 
Development  of  Central  Nervous  System  . 
Origin  of  Mesoderm  ami  Geloin 

Ori^'in  of  the  Notochord 

The  Pra'oral  "  I  lead-cavities  "  of  Amphioxus 
Endostyle  and  Pigment  Granules 

Larval  Diakloi'ment 

Formation  of  Primary  Cill-slits,  etc. 
Formation  of  Secondary  CJill-slits 
Club-shaped  Clland  and  Endostyle 
Continued  Migration  of  Primary  Gill-slits 


rAGi- 
104 

105 
lo<; 

IKS 

120 
IJ4 
126 
129 


130 


IV. 


»35 

13S 


Peripharyngeal  Hands '       o^ 

Atropl,y  of  First  Prin,ary  OiH-sht  and  Club-shaped  Gland,  etc  "     ''''' 
The  Adjustniont  of  the  Mouth,  etc.  . 
Equalisation  of  the  Gill-shts 

Further  (irowth  of  Endostyle,  etc 

Development  of  Reproductive  Organs  .... 

Ge.N'ERAI.   CoNSIDKRATKiNS 

Larval  Asymmetry '     •     ■     '.V5 

Explanation  of  Asymmetry  of  Mouth  and  Gill-slits     "     "     '     "     '" 
l^arval  Asymmetry  not  Adaptive  and  not  Advantageous  . 
Ami'Iiioxis  am.  .\mm()C(i:ii...s  . 

Nervus  Ihanchialis  Vagi 

Stomoda'um,  Hypophysis,  and  Ciill-slits 

Emlostyle  or  Ilypobranchial  tiroove 

Peripharyngeal  Ciliated  Bands  of  .Vmmoccetes   . 

Thyroid  (,;iand 

Morphology  of  Clul)-shai)ed  Oland  of  Amphioxus  .     "     '     ' 
Prawal  "Nephridium  ••  of  Hatschek 
Ancestral  Numiier  of  Gill-slits 

Notes    .     .    . 


140 

143 

14S 

140 

'5' 


^S7 
1 01 

i<J3 

105 
107 
KkS 
1 61) 


CONTENTS. 


Xlll 


FAOl-, 

.      IU4 

•  105 
.      105 

.      109 

•  ".? 
.      IKS 

.  120 

.  IJ4 

.  126 

.  129 

•  130 

•  '35 

•  I3^S 

•  139 

I40 

140 

143 
14S 

14c) 
'5' 

^y:> 
157 

I  01 

i<J3 
"'3 

167 
lOS 
1 61 ) 

ITU 


IV.  THE   ASCIDTANS 

STKirCTlKI'.   OK   A    SlMPI.K    ASCIDIAN 

Test,  Mantle,  Atriuni,  Branchial  Sac 

Dorsal  Lamina,  Endostyle,  and  Peripharyngeal  Band  .     . 

Visceral  Anatomy 

Nervous  System  and  Hypophysis 

Circulatory  System 

Renal  Organs 

Comparison  between  an  Ascidian  and  Amphioxus  .     .     . 

DEVEI.dr.MENT   OK    ASCIOIANS 

.^,  Segmentation  and  Gastrulation 

•-"-  formation  of  Medullary  Tube  and  Xotochord   .     .     .     . 

Origin  of  Mesoderm 

Outgrowth  of  Tail 

Formation  of  the  Adhesive  Papillx 

Cerebral  Vesicle  and  its  Sense-organs 

Comparison  of  Tunicate  Eye  with  liie  Pineal  Eye  .     . 

Stomodaal  and  Atrial  Involutions 

Formation  of  Alimentary  Canal  ami  Hatching  of  Larva. 
Clavelina  and  Ciona 


■# 


Metamoki'iiosis  ok  Ciona  Intestinalis 

Vacuolization  of  the  Notochord 

Mesenchyme  and  Body-cavity 

Pra.'oral  Body-cavity  and  Prajoral  Lobe 

Body-cavity  of  an  Ascidian  and  Civlom  of  Amphioxus    .     . 

Fixation  of  the  Ascidian  Larva 

Reopening  of  Xeuropore;   Degeneration  of  Cerebral  Vesicle; 

I'ormation  of  Delinitive  (.Janglion 

Primary  Topographical  Relations  and  Change  of  Axis     .     .     . 

Formation  of  Additional  Branchial  Stigmata 

First  Appearance  of  Musculature 

Alimentary  Canal  and  Pyloric  ( Hand 

Appcndicuhiria 

Al)l)reviated  Ontogeny  of  Clavelina 

Notes ... 


I'AGE 

iSo 

181 
181 

183 
186 
188 
191 
194 
194 

196 
197 
198 
199 
201 
204 
204 
207 
209 
214 
214 

215 
216 

217 
21S 


220 
222 

223 
226 
229 

235 
235 
236 

239 
240 


!|iil 


XIV 


CONTENTS. 


V.  THE    rUOTOCIIORDATA   IX   THEIR    RELATION   TO  THE 

PROBLEM   OF   NERIEHRATE    DESCENT 342 

Balanocji.dssls 

24- 

External  Features 

"4 

Nervous  System  and  Ouiiads ^ 

-4 

Metamerism 

^4 

15ody-cavities;    I'rohoscis-pore;    C\)llar-porcs      ...  ■•. 

Alimentary  Canal ^^ 

Development;   the  'I'ornaria  Larva ,. 

Tlie    Larva    of   Asterias    \ul,i,'aris;    Water-pores    and    Tra-oral 

Lolie 

Apical  Plate  of  Tornaria 

Metamor[)iiosis  of  Tornaria -,. 

The  Nemertines ^. 

Cki'iialodiscis  and  KiiAr.i)i)i'i.i:rKA „ 

TiiK  Pr.koral  Lome  of  Fciiinodkkm  Lauvi: ,, 

ThK    Pk.KOKAI.    LoI!K    of    Till.;    ri;oT()ril()RI)ATKS j" 

Anterior  and  Posterior  Neurenteric  Canals,  and  the  Position  of 
the  Mouth  in  the  Protoehordates  .  -,- 

TllF.    Pk.TciUAI.    L(i1!|-.    in    niK   CkANIAIK    VFRTKIiKATKS       .       .       .       .      r 

TlIK    Mnrill    ,,K    TiiK    CrnNIATK    VKRIKIiRAlKS 2\ 

SHJNIFICANCK    (IF    TIIK    I  IVi'.  il'H\MS    Cl'.RFliRI 2> 

The  Ascidiaii  Ilvpopliysis ^v 

CiiNtT.rsiON ^y 

NoTi-.s 

21) 

Rk.I  KRKNTF.S 

L<1)KX       .... 

.  ;i 


M 


[OX 


TO  THE 


and 


Tr 


;'a>i 


iral 


•situ 


111 


KS 


242 

24.' 

24 

-^4 


-:4 


INTRODUCTION. 


The  first  zoologist  to  put  forward,  in  a  definite  manner, 
the  view  of  the  existence  of  a  direct  relalionship  between 
Vertebrates  and  Invertebrates  was  the  celebrated  Etiexne 
Geoffrov  Saint-Hilaike. 

It  would  appear  that  without  any  previous  zoological 
training,  haA'ing  been  brought  up  as  a  botanist  and 
mineralogist,  he  was  appointed  Professor  of  Vertebrate 
Zoology  at  the  Museum  of  the  Jardin  des  Plantes  in  the 
year  1793,  being  then  twenty-one  years  old.  His  col- 
league as  Professor  of  Invertebrate  Zoology  was  the  no 
less  distinguished  Lamarck. 

Saint-Hilaire's  study  of  the  comparative  anatomy  and 
osteology  of  the  different  groups  of  Vertebrates  —  P'ishes, 
Amphibians,  Reptiles,  Birds,  and  Mammals --impressed 
him  strongly  with  the  conviction  that,  in  spite  of  the 
many  obvious  contrasts  existing  between  these  animals, 
they  arc  neveriiiv:!-..ss  essentially  constructed  upon  the 
same  plan,  the  same  parts  recurring  in  all  the  groups 
under  a  more  or  less  altered  form.  Moreover,  such 
observations  as,  for  example,  that  the  bones  of  a  fish's 
skull  can  be  more  readily  compared  with  the  bones  of  an 
irnbryonic  mammalian  skull  than  with  those  of  the  adult, 
and  that  the  bones  of  a  bird's  skull  are  separated  in  the 
young  by  sutures  just  as  they  are  in  the  skull  of  a 
mammal,  led   him  to  frame   his  three  great  principles  in 


f 


j0 


i 


2  INTRODUCTION. 

terms   of  which   the   phenomena   of   animal   organisatioiunij 
were  to  be,  to  a  certain  extent,  explained.  lo 

The  three  principles  of  Saint-Hilaire,  each  of  which  ■rat^ 
contains  a  large  element  of  truth,  were  the  following- :  —    rels 

1.  The  Theory  of  Atialogiics,  according  to  which  tin  ted 
same  parts  occur,  in  various  grades  of  form  and  develo]  the] 
ment,  in  all  animals.  asal 

2.  The  Principle  of  Connexions  (Le   principe   des   qo\ 
nexions),  according  to  which  the  same  parts  always  ten    bee] 
to  occur  in  similar  topographical  relations.  cor 

3.  The  Principle  of  the  Correlation  of  Organs  (L.  nou 
principe  du  balancement  des  organes),  according  to  which  anit 
ca'teris  paribus,  the  bulk  of  the  animal  body  remains  ir  its  > 
a  measure  the  same,  and  any  given  organ  can  only  beconii  wha 
enlarged  or  reduced  according  as  another  organ  become^  fuse 
reduced  or  enlarged.  clue 

Having  established  these  principles  in  his  own    miii  legs 
f!'om  the  exclusive  study  of  the  Vertebrates,  the  thouuli:        Ii 

next  occurred  to  him  that  probably  they  were  capable  ni  con( 

equal  application  to  the  rest  of  the  animal  kingdom,  an  hav( 

he    therefore    undertook    the    task    of    identifying  in    th  wor 

Insects  the  typical  structural  peculiarities   of   the  Verti  his 

brates.  anir 

According  to  his  theory  he  would  expect  to  find  in  tin  witl 

Insects,   in    some  form    or  other,   the    same    organs    tlir  be  i 

occur  in  the  Vertebrates.     At  the  outset  he  was,  as  lii  ^"^^ 

successors  have  since  been,   confronted   by  the   palpabK  pru 

fact  that,  while  the  longitudinal  nerve-cord  of  the  Inscc;-  g''® 
lies  next  to  the  ventral  surface  of  the  body,  the  spina 
cord  of  the  Vertebrates  lies  below  the  dorsal  surface 
Accordingly  he  came  to  the  conclusion  which  has  sinu 
been  strongly  advocated  by  the  upholders  of  the  so-calli'. 
"Annelid-theory,"  that  the  "back"    and    "belly"    of  an 


Thi 
cek 

"t) 
ver 


lATKODUCTION. 


lal    organisation  inimal  were  gross   conceptions  of  the  ignorant   and  had 

.10  morphological  meaning.    These  expressions  merely  indi- 

each  of   which  :ated  the  position  which  an  animal  assumed  in  locomonon 

followmg:—    relative  to  the  earth,  and  were  in  this  sense  convertible 

I  to  which  tin  terms,  since  many  invertebrate  animals  prefer  to  swim  on 

II  and  develo]   their  "backs,"  while  some  fishes  also  do  the  same,  others 

again  (flat-fishes,  Pleuronectida3)  swimming  on  their  sides, 
iicipe  des  ctu;  The  surfaces  of  the  body  in  the  respective  groups  having 
ts  always  ten  been  thus  reconciled,  Saint-Hilaire  proceeded  to  a  detailed 
comparison  between  an  insect  and  a  vertebrate.  The  chiti- 
f  Organs  (L,  nous  rings  of  an  insect  represent  the  vertebrae  of  the  higher 
rdmg  to  which  animals.  The  viscera  of  an  insect  are  thus  enclosed  within 
'dy  remains  ir  its  vertebral  column,  and  this  condition  is  compared  with 
n  only  beconii  what  is  found  in  turtles  and  tortoises  where  the  carapace  is 
irgan  become^  fused  with  the  vertebral  column.  It  was  necessary  to  con- 
clude, and  Saint-Hilaire  did  not  hesitate  to  do  so,  that  the 
legs  of  insects  were  equivalent  to  the  ribs  of  Vertebrates. 

It  was  nut  the  intention  of  Saint-Hilaire  to  speculate 

concerning  the  ancestry  of  the  Vertebrates,  for  this  would 

kingdom,  an     have  been  imjjossible  at  the  period  in  which  he  did  his 

:ifying  in    thi    work,  but  he  merely  wished  to  demonstrate  the  truth  of 

of   the  Vertc    his  principle  of  the  unity  of  the  plan  of  composition  of  the 

animal  body.     He  had  therefore  no  reason  to  be  satisfied 

to  find  in  tin    with  having  shown,  as  he  believed,  how  the  Insects  could 

be  regarded  as  possessing  a  structure  essentially  similar  to 

that  of  the  Vertebrates,  hut  he  had  next  to  show  how  his 

principle  could  be  applied  to  other  groups,  above  all  to  the 


lis  own  mill! 
s,  the  though 
3re  capable  o 


organs    tli:r 
le  was,  as  li; 
the  palpabi 


)f  the  Insect-  group  of  the  Cephalopod  Molluscs  (squids,  cuttle-fish,  etc.). 
This  happened  in  the  year  1830,  and  it  precipitated  the 
celebrated  and  somewhat  bitter  dispute  between  the  great 
Cuvier  and  Saint-Hilaire  with  regard  to  the  question  of 
"types."  While  Saint-Hilaire  only  recognised  one  uni- 
versal type,  Cuvier  arranged  the  different  groups  of  animals 


ly,  the  spin, 
5rsal    surfaci 
ich  has  siii.c 
the  so-cal'i ,, 
belly"    of  an 


II  !li 


tli 


Hi 


i 


4  INTRODUCr.ON, 

under   four   entirely   distinct   types;   namely,  Vertebratjei 
Mollusca,    Articulata,    and    Radiata.     Cuvier's  system  (^a 
classif.-cilion  remained  in  use  for  many  years;  in  fact,  uii'Lhei| 
the  progress  of  knowledge  necessitated  the  adoption  of  sim: 
better  one.  ide: 

For  the  first  time,  in  1864,  the  attempt  was  made  !sys 
Levdig  to  grapple  with  the  problem  of  the  origin  of  ti  As 
Vertebrates  in  the  light  of  Darwin's  Theory  of  F,voluli'  int 
(1858).  Singular  to  say,  although  Leydig  approached  t- to 
subject  from  an  entirely  different  point  of  view  from  th  was 
of  Saint-Hilaire,  yet  he  also  attempted  to  find  points  K 
affinity  between  the  highest  Insects  and  the  Vertebratt  cha: 
and  to  identify  the  various  subdivisions  of  the  Vertebra  as  i 
brain  in  the  brain  of  the  bee.  nee 

Leydig  and  all  chose  later  authors  who  would  derive  t  was 
Vertebrates  from  an  articulate  ancestor,  have  started  (  Ko> 
with  the  a  priori  conviction  that  the  segmentation  of  i  reg£ 
body  (metamerism)  which  is  such  a  prominent  feature  (  T 
least  with  regard  to  the  musculature  and  skeleton) 
fishes,  and  can  be  traced  throughout  the  vertebrate  scric 
especially  in  the  embryonic  stages,  is  morphologica 
identical  with  the  familiar  annulation  or  segmentation 
the  Articulates  (Annelids,  Arthropods). 

This  is  obviously  a  very  natural  assumption  to  make.  1 
there  is  a  large  mass  of  facts  which  run  counter  to  it,  sun 
of  which  will  be  referred  to  in  the  following  pages. 

An  unexpected  light  was  thrown  upon  the  problem 
Vertebrate  descent  in   1866,  when  the  Russian  natur-i. 
KowALEVSKV  published  an  account  of  his  researches 
the  embryology  of  Amphioxus  and  the  Ascidians. 

The  Ascidians  or  Tunicates  form  a  curious  and  in  s.tn. 
respects  well-defined  group  of  animals,  which  used  ti*  i 
generally  regarded  as  a  subdivision  of  the  Mollusca  and  :. 


Ver 
Doi 

awa 

disc 

of  1 

sim 

sai( 

the 

] 

vie 

not 

Ar 

inf 

for 


h. 
j,i.-. 


•fiFHaSir^Vi 


INTRODUCTION. 


nely,  Vertebral jcing  closely  related  to  the  section  of  the  bivalves  or 
ivier  s  system  i^ameilibranchiata.  Kowalevsky,  however,  discovered  that 
:ars;  in  fact,  unfLheir  embryonic  development  takes  place  on  a  plan  so 
he  adoption  of  similar  to  that  of  Amphioxus  as  almos^  to  amount  to  an 
identity.  The  development  of  the  nervous  and  respiratory 
pt  was  made  1  systems,  and  of  the  axial  skeleton  or  notochord  in  the 
he  origin  of  ti  Ascidian  embryo,  as  determined  by  Kowalevsky,  showed 
3ry  of  F,volutii  in  the  clearest  manner  that  the  relationship  of  the  Ascidians 
f  approached  t:  to  Amphioxus,  and  through  the  latter  to  the  Vertebrates, 
f  view  from  th  was  an  extraordinarily  close  one. 

0  find  points  Kowalevsky's  discovery  of  the  chordate  or  sub-vertebrate 
the  Vertebrat(  character  of  the  Ascidian  larva,  was  considered  by  Haeckel 
:  the  Vertebra  as  affording  a  direct  solution  of  the  problem  of  the  con- 
necting link  between  Vertebrates  and  Invertebrates.  This 
vould  derive  t  was  a  somewhat  extreme  view  to  take  of  the  matter,  since 
lave  started  (.  Kowalevsky  showed  that  the  Ascidians  could  no  longer  be 
lentation  of  t:  regarded  as  true  Invertebrates. 

nent  feature!  In  1875  the  foundation  of  the  Annelid  theory  of 
id  skeleton)  Vertebrate  descent  was  laid  independently  by  Semper  and 
-rtebrate  scrie    Dohrn  ;    and    Kowalevsky's  observations   were  explained 


morphologica' 
egmentation 

3n  to  make,  1 
nter  to  it,  son 

pages, 
the  problem 
isian  natuni:;- 

rcsearches 
dians. 

IS  and  in  s  u: 
ch  used  ti)  i 
'ollusca  and  .: 


away  in  favour  of  the  new  line  of  speculation.  It  was  the 
discovery  of  the  segmental  origin  of  the  excretory  tubules 
of  the  Selachian  (shark)  kidney,  made  independently  and 
simultaneously  by  Semper  and  Balfour,  which  may  be 
said  to  have  led  to  the  definite  framing  of  the  Annelid 
thepry. 

Dohrn  approached  the  subject  from  a  different  point  of 
view.  According  to  him,  not  only  were  the  Vertebrates 
not  descended  from  forms  allied  to  the  Ascidians  and 
Amphioxus,  but  the  latter  were,  by  a  process  of  almost 
infinite  degeneration,  derived  or  degenerated  from  the 
former. 

That    the    Ascidians   are    degenerate    animals,    to   the 


hXTRODUCTIOX, 


extent  that  they  have  become  adapted  to  a  fixed  habit  o 
life,   is  of   course  obvious ;   but  that  they  have  phyloL;\ 
netically  undergone  the  immeasurable  degeneration  whic 
was   postulated   by   Dohrn,  is   a   view  which   is   entire! 
unjustified  by  facts.     We  shall  now  proceed  to  a  presc 
tation   of    some   of    these    facts,    devoting   the   first   tv, 
chapters  to  the  anatomy  of  Amphioxus,  the  third  to  tl 
development  of  Amphioxus,  the  fourth  to  a  brief  sketch 
the  structure  and  development  of  the  typical  Ascidians,  ai 
the  fifth  to  a  consideration  of  the  more  abstruse  relatii 
ships  of  the  lower  Vertebrates  or  Protochordates. 

The  following  classification  of  the  forms  more  partic 
larly  dealt  with  may  be  of  service  :  — 


ill 


Group.  —  Protochordata. 

Division   i.    Hemiciiorda   (Balanoglossus,  Cephalodiscu 

and  Rhabdopleura.     See  Chap.  V.). 
Division  2.    Urochorda  (Ascidians). 
Division  3.    Cephalochorda  (Amphioxus). 


•  1 

ii 

i 

i 

1 

I' 

f 

! 

I 
1 

t 

i 

k'VKVi 


a  fixed  habit  o 
-y  have  phyln^, 
^^cnoration  whic 
^hich   is   entire! 
■eed  to  a  prcse 
S   the   first   tM 
the  third  to  tl: 
a  brief  sketcJi 
il  Ascidians,  ar 
bstruse  relatii: 
•rdates. 
IS  more  partic. 


Cephalodiscii 
ap.  V. ). 


I. 


ANATOMY   OF   AMPHIOXUS. 

HISTORICAL. 

Tm-:  historical  progress  of  our  knowledge  of  Amphioxus 
has  often  been  told,  but  lor  the  sake  of  completeness  it 
may  be  well  to  sketch  its  main  outlines  once  more. 

It  is  interesting  as  being  one  of  the  few  animals  that 
were  not  known  to  Aristotle,  having  been  described  and 
figured  for  the  first  time  in  1778  by  the  German  zoologist 
Peter  Simon  Pallas.  Pallas  based  his  description  on 
a  specimen  preserved  in  spirit,  which  had  been  sent  to 
him  from  the  coast  of  Cornwall;  and  as  he  confined  him- 
self to  the  examination  of  the  external  form,  he  made 
what  may  appear  to  us  the  somewhat  gross  error  of  re- 
garding it  as  a  Mollusc,  a  species  of  slug,  and  he  accord- 
ingly named  it  lJ})iax  lanccolatns.  He  gives  a  perfectly 
recognisable  figure  of  it,  but  was  led  astray  by  its  flattened 
and  pleated  ventral  surface,  which  might  be  consfued 
iato  bearing  a  faint  resemblance  to  a  Molluscan  "  foot." 

This  not  very  extensive  knowledge  of  Amphioxus  served 
the  zoological  world  for  nearly  sixty  years,  until,  in  1834, 
it  was  discovered  for  the  second  time  in  the  Mediterra- 
nean, bv  the  Italian  naturalist,  Gabriel  Costa.  Costa 
found  it  on  the  shores  of  Posilippo,  in  the  Gulf  of  Naples, 
and  was  the  first  to  make  observations  on  the  living  ani- 
mal and  to  recognise  its  true  nature.     He  thought  at  first 


all'! 


8 


.lAAJOMy   OF  A. ]//'/// OX C'S. 


sai 
At 


wel 


that  he  had  absolutely  discovered  it,  but  subsequently  caiiMst 
across  Pallas's  description.     He  snowed  that  it  was  a  //w^q 
allied  to  the  Cyclcxstomata,   a  group  which   includes  tii 
lampreys  and  ha<;-fishes. 

In  his  account  of  its  habits  he  pointed  out  how  sensitiv 
it  was  to  light,  and  although  without  api)arent  eyes,  yet  tli 
light  stimulated  it  to  such  an  extent  that  it  could  by  v. 
means  tolerate  it.     Costa  mistook  the  curious  tentacle-lik 
processes  or  cirri,  which  form  a  circlet  round  the  moti;    ^o 
(see  Fig.    i,  p.  12),  for  respiratory  filaments  or  branchut    ^^ 
which  suggested  to  him  the  name  of  Branchiostoma  for  t 
genus,  the  specific  name  given    by  him    being  Inbriciim    \^ 
referring  to  the  way  in  which  it  slips  through  the  finger-   \^^ 
with  the  rapidity  of  an  electric  spark  when  touched. 

William   Yakrell,    in   his    History  of   British   Fishc^    poi 
(1836),  was  the  next  to  describe  the  remarkable  creature    j-e 
and  to  give  it  the  name  Amphioxus,  by  which  it  has  becoir,*.    th 
so  well  known  and  which  refers  to  the  fact  that  it  is  pointci; 
at  both  ends.     Yarrell  was  also  the  first  to  describe  ihi 
HOtocJiord  ox  chorda  dorsalis  of  Amphioxus  as  a  cartihigi 
nous  vertebral  column. 

Subsequently  other  observers  had  taken  specimens  o; 
Amphioxus  from  various  points,  notably  from  the  coast  n" 
Sweden,  so  that  the  attention  of  morphologists  was  ,r 
last  definitely  directed  to  the  interesting  form,  and  '\\\ 
1 84 1  there  were  produced  three  independent  memoirs  (wi 
the  anatomy  of  Amphioxus,  whic.  laid  the  foundatimi 
of  our  present  knowledge.  The  authors  of  these  memoii- 
were  John  Goodsir  of  Edinburgh,  Hklxrich  R.xtmkk  "i 
Konigsberg,  and  Johannes  Muller  of  Berlin.  The  work 
of  the  last-named  author  is  a  masterpiece.  With  regaiVi 
to  the  systematic  position  of  Amphioxus,  the  outcome  ft 
all  these  researches  was,  that  it  was  allied  to  the  Cycli- 


hoc 
pai 
po! 
wa 
sei 

(E 

in 

th 

t\ 

CI 

c 


1 


//Alius  AND   D/STKIHUTION, 


bsocjuentlycam>stomata,  but.as  Johannes  Miiller  put  it,  differed  from  them 
1    was  a  /,  w^Q  a  greater  extent  than  a  fish  differs  from  an  Amphibian, 
ch   includes   tii 


Lit  how  scnsitiv 
-nt  eyes,  yc-t  th 
it  could   by  n 
)us  tentacle-lik 
und  the  mou; 
s  or  branchia 
'liostotna  for  th 
>eing  Inbricitin 
».^h  the  finger, 
touched. 
British   Fishc> 
kable  creatine 
1  it  has  beconu 
at  it  is  pointei: 
3  describe  t!u 
as  a  carti)a-i 

specimens  o; 
n  the  coast  oi 
^nsts  was  ;it 
form,  and  w. 
memoirs  on 
t"  foundation 
ese  menioiis 

RaTHKK    III 

The  woik 

U'ith  reijan! 

outcome  o\ 

the  Cyclo- 


HAIUIS  AM)  DISTKIIiUrioX. 

In  consequence  ot  tlie  exteiision  of  the  firm,  and  at  tlie 
same  time  elastic,  notochord  to  the  tip  of  the  snout, 
Amphioxus  i/ossesses  an  extraordinary  capacity  for  bur- 
rowinj;  in  the  sand  of  the  sea-'hore  or  sea-bottom.  If  an 
individual  be  dropped  from  the  hand  on  to  a  mound  of 
wet  sanil  which  has  just  been  dred^^ed  out  of  the  water, 
it  will  burrow  its  way  t«)  the  lowest  depths  of  the  sand- 
hillock  in  the  twinkling;  of  an  eye. 

The  frontispiece  is  dcsit^ned  to  illustrate  the  chief 
positions  in  which  Amphioxus  may  be  observed.  It  is 
represented  swimming,  lying  on  the  sand,  and  buried  in 
the  sand. 

Its  usual  modus  viviiuii  is  to  bury  the  whole  of  its 
body  in  the  sand,  leaving  only  the  mouth  with  the  ex- 
panded buccal  cirri  protruding.  When  obtained  in  this 
position  in  a  glass  jar  a  constant  inflowing  current  of 
water  in  which  food-particles  are  involved  can  be  ob- 
served in  the  neighbourhood  of  the  upstanding  mouths. 

The  food  consists  almost  entirely  of  microscopic  plants 
(Diatoms,  Desmids,  etc.)  and  vegetable  debris. 

While  passing  through  the  pharynx  the  food  becomes 
involved  in  the  slimy  secretion  of  a  gland  at  the  base  of 
the  jiharynx  known  as  the  ctidostylc  or  hypobrancJiial 
groove  (cf.  Figs.  2  and  3),  and  is  thus  held  in  the  pharynx 
while  the  water  with  which  it  entercl  flows  out  through 
the  gill-slits  into  the  atrial  chamber.  The  food  is  then 
carried  through  the  intestine  enveloped  in  a  continuous 
cord  of  slime  or  mucus,  which  is  kept  in  perpetual  motion 


10 


,LV.I/0.)/y   01-   ,lM/'///OXi\S. 


'  If 


I'  • 


1   i!!M 

i  ■' 


and    rotation    by  tlio   action  of  the  cilia  with  which  tl 
epithelium    of    the    alimentary    canal    is    richly   provide 
After  the  digestible  elements  in  the  food  have  hcomi 
solved  in  the  secretions  of  the  intestinal  wall  ilu'  cord 
slime  with  the  attached  f;eces  is  duly  ejected.-* 

The  extreme  shyness  to  a  bright  or  sudden  lit^ht  whi, 
as  C\)sta  observed,  is  manifested  by  Amphioxus,  is  pi 
ably  correlated  with  the  presence  of  black  pi-imiu  >; 
in  the  nerve-cord.  If  a  lighted  candle  is  carried  iiu 
dark  room  in  which  Amphioxus  are  being  kept  in  ;', 
jars,  the  excitement  produceil  among  the  small  tish 
indescribable. 

Occasionally  it  emerges  from   its   favourite  position 
the  sand,  and  after  swimming  about  for  some  time  it  w 
sink    to    the    bottom,  and   there    recline   for   a    lon-cr 
shorter  period  ujion   its  side  on  the  surface  of  tin.'  s.ir 
When  resting    on  the   sand,   it    is  unable  to  maintain  ; 
equilibrium  in  the  same  jiosition  as  an  ordinary  tish  wo:: 
do,  but  invariably  tojiples  over  on  its  side,  indifferently 
the  right  or  left  siile.''     In  the  higher  fishes,  inchulingt: 
lampreys,  there  is  a  sj)ecial  apparatus  for  controlling  t: 
equilibrium;  namely,  the  semicircular  canals   of  the  c 
There  is  nothing  of  the  kind  in  Amphioxus,  but  in  t: 
Ascidian    larva   and  in   the   Appendiculariir    there   is, 
we  shall  see,  a  structure  situated  in  the  floor  of  the  bn: 
known  as  the  otolitit,  which  possibly  exercises  an  eqiiil;: 
rating  influence. 

iMom  what  has  been  said  above  it  follows  that  Amph: 
oxus  is  an  entirely  passive  feeder ;  it  does  nothing  in  th; 
way  of  biting,  or  even  sucking,  and  has  not  to  search  fa; 
tor  its  food,  but  merely  takes  what  is  brought  in  with  tb 

♦  This  nuinl.(.T  an.l  ..iIrts  wliicli  are  stxittered  through  tlie  Uxl  uUr  toth 
Notes  at  the  ends  df  the  chapters. 


ci| 

tl 

si 
t( 

iti 

tl 

S( 

b 


I 


u  -.v. 

I  in  with  whi^'h  ti 
•'^    '■'^'•I'y   pn.vid, 

il  w;ill  tlu-  cord 
IT  ted.-  • 

luldcii  li^dit  whi. 
"iI^'^ioMis.  is  pr 
K-'k  i)i-nirnt  ,s[- 
is  carnrd  int- 
'in-;-  kept  111  ^: 
tlic    .small    lish 

^uritc  position 
some  time  it  w 

for  a  lon-xT 
face  ot"  the  sar 
i-'  to  maintain  : 
diiKiry  fish  wo;. 
.  indifferently 
cs,  includin!:!: 
r  controllinc;  tr 
tials   of  the  C2 

xus,  but  in  t: 
iiv    there   is. 
oor  of  the  bra 
:ises  an  eqiiili: 

\vs  that  AmpL 

nothin-;-  in  th; 

ot  to  search  far 

[i^ht  in  w  ith  the 

the  tf\t  u  trr  totr.f 


UAUirS  AND   DISTKIHUTIOX. 


It 


water  which  is  drawn  into  the  mouth  by  the  powerful 
ciliary  action  of  the  cells  lining  the  roof  of  the  mouth  and 
the  wall  of  the  pharynx. 

Speaking  generally,  Amphioxus  is  an  inhabitant  of 
shallow  water;  it  is  essentially  a  littoral  form,  and  is  apt 
to  occur  in  the  neighbourhood  of  any  sandy  shore.  Its 
occurrence,  however,  is  often  curiously  local,  as  shown  by 
its  behaviour  at  Messina.  In  the  vicinity  of  Messina 
there  are  a  couple  of  rather  extensive  salt-water  pools,  at 
some  points  of  considerable  depth,  which,  in  the  course  of 
ages,  have  apparently  been  shut  off  from  the  adjacent  sea 
by  the  formation  of  sandbanks.  In  the  more  northerly  of 
these  small  lakes,  lying  almost  at  the  extreme  north- 
eastern point  of  Sicily,  Amphioxus  occurs  in  astonishing 
abundance  ;  while  in  the  more  southerly  lake,  which  is 
connected  with  the  former  by  a  narrow  artificial  canal,  it 
is  entirely  absent,  lioth  of  these  lakes  communicate 
by  narrow  outlets  with  the  Straits  of  Messina,  where, 
however,  Amphioxus  is  somewhat  rarely  met  with.  In 
the  Gulf  of  Naples  it  is  extremely  abundant ;  while  in 
Plymouth  Sound,  in  the  English  Channel,  it  is  compara- 
tively rare.  On  the  coast  of  France  it  is  said  to  grow  to 
an  unusually  large  size.  It  has  been  taken  in  greater  or 
less  numbers  from  many  other  localities  in  Europe,  on 
the  Atlantic  and  Pacific  shores  of  North  and  South 
America,  and  from  the  shores  of  Australia,  Japan,  and 
Ceylon.  Its  geographical  distribution  may  therefore  be 
said  to  be  pre-eminently  world-wide,  and,  in  fact,  it  is 
liable  to  turn  up  on  any  shore  in  the  temperate  and 
tropical  regions.  And  yet  with  all  this  world-wide  distri- 
bution there  is  only  a  single  genus,  with  some  eight 
species,^  the  different  species  being  remarkably  alike, 
differing  slightly  in  the  height  of  the  dorsal  fin  and  in 


W' 


12 


■'■''■"-^'^^y  oy-  .,.uryyyo.vrs. 


'I    i'    :^ 


'II     i! 


tilt,'  number   of  mu<r]n  ^ 

"-'»"'  foa.,„...    ,„       ;  •"'  f"   »■'"'    it.s    ,,,„„t 
enccs,   ,si,„u-.    that    w.    Inv.  ,         ""    ■^''^■'•"i<'- 'ii'l 

^"■chaic  lorn,.  '""    '"   ''"    "'itl'    an    .,t,x.„, 

'■^ |'i:k.\.\i.  |., imi 
•-\  Koud    idea  „f    tliL.   ,...,, 

f'"'^ni   about    four  t 

"'^'  '■•-''  -n.liti.,,;' if, '"'"''.■■'■'    ""'"    '-■-"timer...    I: 
.„■  ti,^.  .„,^._.^^,^|  ^^^^^^^^^^      s  ^em,.tra„,sparc„t.  s„  that  ,,o„: 

'■^  _"'"''■"  inVlcsc.nt"!     '  ''■'■"  ""■""«■''  t''-  ski".  ui,,v 

J'ii^'  ti,i;iir..  shows  th,.  „  •  .    , 

'"■■•'"■^-  the  n,o„th  ""^.     ^''■""■■''t-'y.    the   „,,/  ,„„„ 

"'" "'"  "'^'  ""^■-"' '"t  , ; :;""""  '"•^''-  ">•  -^  i^ ■'■^- 

i'roco.ssos   .-row  „„t       k,'       •       '  '"""'"  "^  >^l'icli  H-h" 
^«'--n>ity  of  tlr.  ,,.„,,  „;',';'"'"'-    '>""'   "car  the  am.,., 

'""■  '"tch  hent  upon   its.     i„ 


if 


.\7  X 

^i  K-ivcn  species, 
■'""  ''f  A.nphio, 
'"■^'-^Iwellerand; 
5'i  its  remark:,: 
'".^'  ■^pceitie  (!iit. 
■'^'^    ^i"    cxtreiiK 


EXTERNAL    EORM. 


n 


''"ce  an  [imj 
''stained  tiom  : 
ual    Iciii^tli  var 


"""  twice  lutuMls: 

"lS[l,Ufllc\-  ihiouoh- 
'  I'C'lou-  the  iiu'l.ip":!.::: 
'•n  ;iti-ir)[,orc,in,l,iir: 


entinictfes,  1; 
-  "^o  that  sen: 
the  skin,  \s\vx. 

-s  of  the  !)(),: 
o'^ind  the  mar 
•he  oral  Itooii 
by  a  h()()(l-!ikc 
f  which  ih-^se 
the  ant.'rior 
re  seen  -^nic 
ipon    its;  ■  in 


such  a  way  as  to  form  two  sides  oi"  a  triangle,  the  apex 
of  which  is  directed  forwards.  These  are  the  partitions 
or  septa  which  divide  the  longitudinal  muscles  of  the 
body  into  a  series  of  separate  muscle-chambers  or  )nyo- 
tomes.  In  virtue  of  the  longitudinal  muscles  being,  broken 
up,  so  to  speak,  into  a  great  number  of  segments,  the 
animal  is  enabled  to  swim  rapidly  with  a  serpentine 
motion.  In  the  remarkable  pelagic  animal,  Sagitta,  where 
the  muscles  are  not  segmented,  this  motion  is  impossible, 
and  instead,  it  darts  forward  by  sudden  and  spasmodic 
jerkings  of  its  tail. 

In  Amphioxus,  the  tail  or  post-anal  region  of  the  body 
is  very  much  reduced,  and  the  muscle-segments  of  the 
trunk  therefore  constitute  its  only  means  of  locomotion, 
there  being  no  muscular  fins.  Beyond  the  muscle-plates, 
both  in  front  and  behind,  the  notochord,  which  forms  the 
axial  skeleton  of  the  body,  is  seen  to  extend  to  the  anterior 
and  posterior  extremities.  The  extension  of  the  notochord 
beyond  the  anterior  limit  of  the  dorsal  nerve-tube  is  a  very 
exceptional  condition,  and  has  led  to  the  creation  of  a 
special  order  for  the  reception  of  Amphioxus  ;  namely,  the 
Cephalochorda. 

The  oval  structures  seen  lying  below  the  muscle-plates 
in  Fig.  I  are  the  reproductive  organs,  male  or  female  as 
the  case  may  be.  Instead  of  being  represented  by  a  single 
genital  gland  on  each  side  of  the  body  as  they  are  in  the 
higher  fishes  and  Vertebrates  generally,  they  consist  here 
of  some  twenty-six  pairs  of  perfectly  distinct  chambers, 
occurring  in  correspondence  with  the  muscle-segments  or 
myotomes  of  the  region  to  which  they  belong,  and  extend- 
ing from  the  tenth  to  the  thirty-fifth  myotome  inclusive. 
These  chambers  are  known  as  the  gonadic  pouches.  (See 
Fig.  2.) 


/■ 


14 


AX.i/o.yy  OF  AMri/ioxis. 


About  two-thirils  oi  the  way  from  the  front  ciul  of  t: 
boilv  there  is  a  comixirativcly  lari;e  aperture  in  the  n„ 
ventral  line.  It  is  the  excurrent  oritiee  of  a  spacin 
cavity  which  surrounds  to  a  lari;e  extent  the  intcn 
or_<;ans,  inchulin-;'  abcn-e  all  the  pharynx,  antl  is  known: 
the  atrial  cJiavibcr,  or  simply  atrium^  w^ile  its  openini; : 
the  exterior  is  the  atrioporc. 

The  a}>Hs  or  outlet  of  the  digestive  tract  occurs  noartr. 
posterior  end  of  the   body  ;    it  does  not   lie   in  the  rai. 
ventral  line,   but  hi!j;h  up  or.   the  left   side.     At   its  tir: 
appearance  in   the  youn;^  embryo,  the  anus  tloes  lie  a: 
proximately  in  the  mid-ventral  line  (cf.   Fi;;".  64,  p.  11; 
but  as  soon  as  the  caudal  fin  be_<;ins  to  develop,  it  is  puslu 
on   to  one  side,  always  the  left,  and  so  attains  its  fin. 
position.     A  similar  displacement  o^"  the  cloacal  apcrtii:: 
occurs  in    the  Dipnoan  fish  Protoptcrus,  where,  howcw 
the  direction  of  displacement  is  not  constant,  the  apcrtur: 
lyinj;  now  to  the  rij;ht,  now  to  the  left,  of  the  middle  lin. 
Again,  in  the  tadpoles  of  certain  Batrachians  the  cloaci 
aperture  is  displaced  to  tl.e  right  of  the  middle  line.*-    iC: 
Fig.  8.)     The  fact  of  the  displacement  of  openings  by  th; 


di| 

01 

si] 


bi 

P! 


i  1 


*  The  asymmetrical  pusitinn  of  the  cloacal  aperture  of  certain  Hatr.ulr.a: 
tadpoles  has  been  systematically  worked  out  !n-  liofl.KNiir.K.  In  t.vlpili- 
ot  the  ,i;enera  Rana  and  Hyla,  the  cloacal  ajierturc  is  dcxtral,  wliilc  in  tb 
Toads  and  I'elubatoiils  it  is  median.  (See  ('..  A.  lioflKNCKK,  ./  Svnofi. 
of  the  Tadpoles  of  ihe  Eiuopcait  Hati-iuhians.  I'roc.  /,ool.  .Soc.  I  .mmc 
1S91.     pp.  593-6,-!7.     Plates  45-47.) 

In  kana  tlie  cloacal  ajierturc  may  occasionally  occur  in  a  median  jMsitiic 
as  a  variation.  (Wii.i.i.v.  Xotc  on  ihc  fosition  of  tlw  obacal  ,7/,r.'.v'Y  n: 
drtain  Ilihuichian  A/u/o/cv.  Transactions  \ew  York  .\cad.  of  Sciences,  Vol. 
XII.  iSg;,.  pp.  242-J45.)  My  attention  to  the  previous  literature  >»  thi- 
subject  was  kindly  drawn  by  Mr.  ( "..  .\.  lloulenj^er. 

Since  writini;  tlie  abi.ve  my  attention  has  been  called  to  the  U'v-  uin; 
paper  by  Professor  lliui  C.  Wii.i.KU,  /.,ihr,il  J'osition  of  the  Vent  lu  Am- 
rhio.Mis  [P.ranchiostomaJ  ./;/,/ ;;/  the  I arvw  of  Rana  /'/>'>'"  [Catesbiina]. 
Proc.  .\mer.  .Assoc.  .\dv.  Sc.  XXII.     1S73.     pp.  275-300. 


t 
S 
< 


V.,^- 


/ 


Y  -.v. 

-  ^''ont  ciul  of  t.- 
-'■turc  in  the  ni: 
^^^  *>'"  a  spacio: 
:t^^"t  the  inter, 
^"(I  is  known- 
-'-   its  oncnin<f- 

•t  occurs  near  til: 
Ii<-'  ill  the  mi. 
de.     At   its  tir; 
nus  does  lie  a; 
I^^ii;-.  64,  p.  I,- 

L'J"P.  it  is  puslk. 

attains  its  fin,; 

cloacal  apcrtnr: 
^vliere,  howcvc: 
"t,  the  ajicrtiir. 
■he  middle  lin. 
ans  the  cloaci 
idle  line.'-  (C: 
'penings  by  ti;: 

^<^rtaiii  i;ntrach;j: 
•'•'■:'<•  In  t.hljiiilf 
extral,  whilr  in  i[ 

'ool.    Sue.    f.umk 

'1  median  ]i.)sitii)r 

odta/  apnlure  in 

1.  of  Sciences.  Vol. 

literature    ai  this 

I  to  the  Ail!,nvinj 

'''«■?  [Catesi,i.inaj. 


EXTERNAL  FORM. 


15 


differential  growth  of  neighbouring  structures  is  a  very  curi- 
ous one,  and  sh*^  uld  be  borne  in  mind.  It  will  have  a  special 
significance  wh'^n  we  come  to  consider  the  development. 

There  are  no  paired  muscular  fins  in  Amphioxus,  but 
running  along  the  whole  length  of  the  back  is  a  median 
ridge  which  is  called  the  dorsal  fin.  It  extends  round  the 
front  end  of  the  body,  where  it  becomes  continuous  with 
the  right  half  of  the  oral  hood.  (Cf.  Fig.  9.)  Posteriorly 
it  becomes  enlarged  to  form  the  tail  expansion  or  caudal 
fin,  and  is  continued  round  the  hinder  extremity  of  the 
body  past  the  anus  as  far  as  the  atriopore.  Along  the 
back,  this  continuous  fin  is  supported  by  a  series  of  gelat- 
inous fill-rays,  each  of  which  lies  in  a  chamber  of  its  own. 
The  fin-rays,  whose  number  may  exceed  250,  do  not  extend 
to  the  extreme  anterior  and  posterior  ends  of  the  body. 
The  ventral  portion  of  the  fin  in  the  region  between  atrio- 
pore and  anus  is  supported  by  a  similar  series  of  fin-rays, 
but  there  are  two  of  them  placed  side  by  side  in  each  com- 
partment. In  other  words,  the  fin-rays  of  the  ventral  fin 
are  paired. 

Amphioxus,  like  most  fishes,  is  laterally  compressed  so 
that  a  transverse  section  through  the  body  in  front  of 
the  atriopore  is  found  to  have  the  form  of  an  equal-sided 
spherical  triangle,  the  base  of  which  consists  of  the  floor 
of  the  atrial  chamber.  At  each  of  the  basal  angles  of 
the  trianfile  there  is  a  fold  of  the  integument  containing 
a  cavity  (Fig.  2).  This  is  the  mctaplcnral  fold^  which 
stretches  on  each  side  of  the  body  from  the  region  of 
the  mouth  to  slightly  beyond  the  atriopore.  (Cf.  Fig.  i.) 
The  cavity  in  the  folds  is  the  inctaplciiral  lynipJi-space. 
The  apex  of  the  triangular  cross-section  is  formed  by 
one  of  the  dorsal  fin-chambers  enclosing  a  lymph-space 
into  which  a  fin-ray  is  projecting. 


J'l 


i6 


-i^■r/■cwy  of  AMrnioxcs, 


vX 


il 


^  Jiu.   K^rth^i;""'  "^'"  '^'""  i"  <."n:^^     "":    .<-:   Monadic  po„ol,  n.n,.>: 

''O'.    I.on./i,„;iin^,  ^*  '"'"■■'"^''i^^l )    ffroove.       /,    '  r  '"''  '    ^"•"'  ""'  '"'ffht  aorta  hv  .hi 
muscles  ;7  """"'"'    "f  mvotnm,.s-   ov'-r  ''""^'"'-^P-^^-^'-       '''A     Metaplec:, 

muscles      -,„    v  ^-   ',' '^'">'"^-     '••  I':x-'-.vt..rv  t,,      i  =i'"I"minis  of  Sdin.uk 

N.M    T      ■;:"":'  '"'"  ">'••  n^-cl..-f^f;;;""  «'"-*---^  ^>f  -'^ich  have  the  app.-a:- 
-J  u(    connective   fisci,,,  /       . 

^^'^•'  '^  'nclicate.,  I,.  „„.  u.eulnlr""'  """^'^''"^"^'   ^-th.  cComic   epi.heli., 


J>\ 


EXTERXAL  FORM. 


17 


dn 
ly 


-hv 


t-il  rpfjion  (if  iVra;- 
rtwijj's  I.ehrbich  . 

ivity  by  tlu'  .Idiil* 
cli.  N'otiii  Im-; 
ostylar  i-(x>I(>tii  ui;- 
ulic  poiu'li  ((inf.!:- 
:»'  which  sninnir!; 
ripht  aorta  hv  the 
mp.  Mftaplei;:. 
irsal  rij/lom  fliese 
linis  of  Sclincidor, 
svltsc  or  siibatria: 
1  have  the  anii'-ar- 

lomic   epithelium 


In  young  transparent  individuals,  such  as  that  of  which 
the  anterior  portion  is  represented  in  Fig.  3,  the  pharynx, 
or  first  division  of  the  digestive  tract,  into  which  the 
mouth  leads  directly,  can  be  seen  through  the  body-wall, 
and  it  is  found  to  be  perforated  on  each  side  by  a  great 
number  of  elongated  vertical  slits,  whose  number  varies 
with  the  age  of  the  individual,  but  may  eventually  attain 
the  asconishing  figure  of  180  pairs.  They  are  the  gill- 
clefts  opening  from  the  pharynx  into  the  atrial  chamber. 
In  the  living  Amphioxus  an  almost  continuous  stream  of 
water  is  being  drawn  through  the  mouth  into  the  pharynx 
for  purposes  of  respiration  and  nourishment,  then  pass- 
ing out  of  the  pharynx,  by  way  of  the  gill-clefts,  into 
the  atrial  chamber  and  thence  to  the  exterior  through  the 
atriopore. 

Cranium  and  Sense-organs. 

Besides  lacking  differentiated  lateral  fins,  Amphioxus 
differs  fundamentally  from  the  higher  Vertebrates  in  the 
absence  of  a  cranium,  of  paired  eyes,  and  paired  or  un- 
paired auditory  organs. 

On  account  of  the  absence  of  a  cartilaginous  cranium 
it  has  been  placed  by  itself  in  a  separate  division,  the 
Acrania,  in  contrast  to  all  the  other  Vertebrates  proper, 
from  the  Cyclostomata  ui)wards,  which  all  possess  a 
cranium  of  one  sort  or  another  and  are  hence  known  as 
the  craniate  Vertebrates  or  Craniota.  In  Amphioxus  the 
only  cartilage  in  the  head-region  consists  of  a  ring  lying 
round  the  margin  of  the  oral  hood  at  the  base  of  the 
buccal  cirri.  It  is  formed  of  separate  pieces  correspond- 
ing to  the  number  of  the  cirri,  and  each  piece  sends  up  a 
process  into  its  adjacent  cirrus,  so  that  the  latter  is  pro- 
vided with  a  stiff  skeletal  axis  (Figs.  3  and  4).     These  are 


l8 


./.\'./y'tu/i'  oj-  AMrj/ioxi's. 


it 


Fig.  3.  —  Anti-rior  portion  of  l)ody  of  yoiini; 
tr,in>ii,iiciu  indivkiuai.  (After  J.  Mi  l.l.KK, 
sliglitly  altiTccLi 

,•'/.  N'otoelioni.  ./.  HiK'i.iil  cirri.  (•.  l'>i'- 
spiit.  r>iil.  l-'nilostyK'.  i.i .  I''in-r;iys  lyiiii;  in 
tlif  t'in-ch;inilit.'rs.  i^'.i.  (iill-Mits;  tlic  skt'lctal 
rods  of  the  i^ill-hars  arc  indioatcd  by  b'ack  liiifs. 
rit.  Spinal  ford,  witli  pii^nicnt  L;rami!t'-;  near  its 


the  bncca/  Ciirtilni^is.     As  pointed  out  by  Johannes  Mu. 

tiiey  arc  not  to  be  compared  with  the  jaw-apparatus,: 

to  the  hyoid  or  ton 
bone  ot  the  jaw-bear; 
Vertebrates,     but   t!-. 
belont;  to  the  same  a: 
_i;ory  as  the  nioulh-ca: 
hiLTcs  of  the  Cvclostn: 
fishes  (which  possess 
hyoid  cartilage  in  at; 
tidii )  and  the  labial  r 
iiidi^cs      of      Selach:.:: 
(sharks). 

The  absence  of  pair. 
eves  antl  of  an)'  kiini 
auditor V  or>;an  has  b« 

ba>-.     ;■.,;.  I  Jowni^rowlli  from  rii;lit  aort.i  lyins;  nicntioncd  aboVC.    Tilt: 

to  till'  ri'  lit  ot  tr/.  liu' velum  ;  with  velar  ten-  .  . 

tacles  projecting;  back  into  i)harynx.     t.-..'.  Kad-  'S,     nOWCVCl,     a     IlK'th, 

eror^an ;     ciliated    epithelial    tracts    on    inner  .....>     which   eonsistS  (' 
surf.ice  of  oral  houd.  -      ' 

comparatively  lar^e  .. 
paired  pigment  spot  lyins  at  the  anterit)r  extremity  of : 
dorsal  nerve-tube.*  A  row  of 
similar,  but  much  smaller, 
masses  of  pio;ment  lie  aloni; 
the  floor  of  the  spinal  canal, 
commenciuL;'  some  distance 
behind  the  eye  (b'ii;.   3). 

Immediately  above  and  be- 
hind  the   eve-spot    is    a   small   r''T'"^'V'''  -'i  ;^'''';''';h..m 

'  basal  |)ieces  he  end  to  end  in  tlicitu 

pit    in    the    botly-Wall     reachini;'  .i,'in  of  oral  hood,  and  each  b.isal  pi^ 

-  ,  r      1  '  >cnds    UP  an   axial   i)rocess    into  !- 

trom    the    outer    surface    of    the  c..rres|.onding  buccal  cirrus. 

*  The  eye-spot  lias  been  oliservcd  to  be  so    .etimcs  broken  up  into  t' 
pigmciU  masses.     (See  AVKKS,  No.  105  biblioj;.) 


Fig.  4.  —  Buccal  cartilaKc.-.  of  .A 


hoc 

th( 

as 

aftj 

cell 

carl 

it 

dej 

tioi 

of 

has 

ap 

por 

lat( 


EXTERNAL  EORM. 


19 


ohanncs  Mu^ 
^'-apparatus, : 

lyoid  or  ton^ 

the  jaw -bear; 
itcs,     but   tr 

;)  the  same  I., 
the  moiith-Q: 
the  Cyclost  • 
I'hicli  posscjv 
.rti]ai;e  in  a.: 
1  the  labial; 
of      Selach:.,, 

>sence  ot  pa;:. 
of  any  kiiii; 

i>rL;'an  has  b.. 

d  above.  TIk 

.'er,  a  iiicdi. 
1  eonsisls  1: 
vely  laii;!'  :; 

s:t  remit  \-  oft: 


al  cartilafj (.■.-,  of  A' 

Mi'I.I.KK.'l  1 
1  ti)  ftui  in  liiciii.^ 
11(1  fai'h  l).i>.ii  piiJ 

process   into  V 
:i:al  cirrus. 

ukcn  up  ir.ti)  t» 


body  to  the  anterior  wall  of 
the  brain.  This  is  known 
as  Kollikcrs  olfactory  pit, 
after  its  discoverer.  The 
cells  which  line  its  walls 
carry  long  vibratile  cilia,  and 
it  possibly  subserves  in  some 
degree  an  olfactory  func- 
tion. In  the  larva  the  cavity 
of  the  brain  opens  into  the 
base  of  the  olfactory  pit  by 
a  pore  known  as  the  neuro- 
porey  which  we  shall  consider 
later.  In  the  adult  this 
pore  becomes  closed,  but 
the  base  of  the  olfactory  pit 
appears  to  remain  connected 
with  the  roof  of  the  brain 
by  a  solid  stalk.  The  olfac- 
tory pit,  like  the  anal  open- 
ing, lies  asymmetrically  on 
the  left  side  of  the  body 
(Fig.  5).  It  is  forced  to  one 
side  in  the  course  of  the 
development  consequent  on 
the  formation  of  the  fin-like 
expansion  of  the  integument 
in  this  region,  which,  as  we 
have  seen,  is  nothing  more 
than  the  cephalic  continua- 
tion of  the  dorsal  fin. 

The  mouth  of  Amphioxus 
would     seem     to    be    well 


Fig.  5.  —  Transverse  section  through 
region    of    olfactory    pit.      (.After    L.\N- 

KKSTKU.) 

The  olfactory  \)\X  is  seen  as  an  ecto- 
dermic  involution  on  the  left  side  in  con- 
tact with  the  wall  of  b,  the  cerebral  vesicle. 
ch.  N'otochord.  f.  Lymph-space  of  ce- 
phalic portion  of  dorsal  fin.  r.k.  anil  IJi 
Right  and  left  portions  of  oral  hood. 
my.  Muscles  of  first  myotome;  outside  of 
the  muscles  is  the  inyoc(L'lic  lymph-space 
of  tirst  myotome;  inside  of  the  muscles 
is  the  apex  of  the  myociulic  lymph-s|iace 
of  the  second  myotome.  ;/.  Cranial 
nerve  (second  pair). 

X.B.  —  The  dotted  shading  represents 
the  thickened  gelatinous  connective  tissue 
of  the  head-region  in  which  irregular 
lymph-spaces  occur. 


20 


Lv.r/o.uv  01'  AMriiioxcs. 


_<;uanlctl  against  the  intrusion  of  noxious  substam 
ICvcrvthing  entering  the  mouth  has  tt)  pass  Ihrou^; 
vestibule  richly  jirovided  with  sensitive  epithelial  a 
This  vestibule  consists  of  the  oral  hood  with  its  niari:;- 
eirri,  at  the  back  of  which  lies  the  definite  oral  opcniii, 
I'ilinu,  as  it  was  called  by  1  Ii'.\Li:v  on  acci-uut  o! 
resemblance  to  a  similar  structure  in  the  young  lamp- 
(Ammoc(etes).  (Cf.  l-'ig.  3.)  In  the  adult  the  \V.. 
carries  twelve  tentacles  of  its  own,  the  viliv  tcit'uii 
which  are  not  to  be  confused  with  the  lutccal  cirri  q{\ 
oral  hood.  The  velar  tentacles  jiroject  in  a  backw.. 
direction  freely  into  the  pharynx. 


Si 
vel 

anl 
mJ 
bel 
dt 

b3 
orl 


;» 


m. 


Fig.  6.  —  ./.  Portion  of  a  buccal  cirrus  to  show  groups  of  senso-cclls. 
/>'.  Isolated  ci'lls  ot  tile  ^kin;  \\\o  cohiuinar  sense-ceils  carrvuii;  a  si'ii-iy!.' 
and  one  cylindrical  eiiulerniic  cell  with  striated  cuticular  border.      (.Ml'  i  l- 

(li;  KUANS.) 


Groups  of  sense-cells  occur  on  the  side  of  the  Imcc: 
cirri  at  intervals  (Fig.  6).  Some  of  these  cells  bc>^r . 
vibratile  cilium  at  their  free  ends,  and  others  bear  sf- 
hairs.     Both  kinds  of  cells  arc  mingled  in  the  same  grou; 


EXTERNAL  FORM. 


21 


iDUs  suhstan. 
'   pass  I  h  roil:; 

epithelial  a- 
'ith  its  inapT 
•  oral  opcniii, 
1    acci'uiit  ()! 

youii-  lamp: 
iluit    the   \L 

velar  ti'ii!i\{. 
iccal  cirri  ol  :. 

in    a  hackuv 


iense-colls. 
order.      (Aliri  \.\ 


of  the  biicc: 

cells    bupr . 

lers  bear  sti: 

ic  same  cron;' 


u.f 


5^,i:.yrJ/I 


Fig.  7. 


Velum  of  Amphioxus  seen  from 
(After  Lankks- 


the  inside  of  the  pliiirynx. 
TKR.) 

v.sp.  Sphincter  muscle  of  velum,   v.t.  Velar 
tentacles  lying  across  the  oral  opening. 


Similar  groups  of  sensory  cells  occur  on  the  margin  of  the 
velum  and   ii  .   tentacles  (Fig.  7).      It   may  be  noted,  in 
anticipation,  that  the  velum   is  derived  directly  from  the 
mouth  of  the  larva,  which 
becomes  secondarily  hid- 
den from  superficial  view 
by  the  overgrowth  of  the 
oral  hood. 

According  to  Langkr- 
HANS,  similar  cells  to 
those  mentioned  above, 
carrying  stiff  sensory 
hairs,  are  scattered  dif- 
fusely all  over  the  exter- 
nal surface  of  the  body. 
(Cf.  Fig.  6  B.)  But  a 
concentration  of  sense- 
organs  comparable  to  the  lateral  line  of  the  higher  fishes 
is  apparently  absent." 

A  remarkable  structure  which  seems  to  combine  the 
properties  of  gland  and  sense-organ  occurs  on  the  under 
surface  of  the  oral  hood.  It  consists  of  a  patch  of 
modified  epithelium  drawn  out  into  finger-shaped  epi- 
thelial tracts,  the  cells  of  which  carry  long  cilia.  (See 
Fig.  3.)  It  was  discovered  and  accurately  described  by 
Johannes  Miiller,  who  called  it  the  "Raderorgan"  on  ac- 
count of  the  resemblance  of  its  ciliary  movements  to  those 
of  the  wheel-apparatus  of  a  Rotifer.  The  result  of  the 
combined  action  of  the  cilia  is  to  cause  a  flow  of  water 
into  the  pharynx.  In  connection  with  the  Riiderorgan 
must  be  mentioned  a  special  depression  forming  a  peculiar 
sense-organ  (Geschmacksorgan)  lying  against  the  right 
side  of  the  notochord,  known  as  the  groove  of  HatscJick. 


22 


AXATOMY   01'  AMPIIIOXUS. 


INTERNAL  ANATOMY, 


Atrial  Cavity. 

In  making  a  dissection  of  a  frog  or  a  fish,  as  soon  as  the 
body-wall  is  cut  through,  we  find  ourscU-es  groping  about 
in  a  large  cavity  in  which  the  viscera  lie.  This  is  the 
body-cavity  ox  peritoneal  cavity^  or,  again,  the  calom. 

If  we  slit  open  the  ventral  body-wall  of  Amphioxus,  we 
discover  what  appears  to  be  an  exactly  similar  cavity.  It 
is,  however,  not  the  ccelomic  cavity,  but  the  peyibrancliial 
or  atrial  cavity,  into  which  the  pharyngeal  gill-slits  open. 
The  older  anatomists,  including  Johannes  Mullcr,  regarded 
it  as  the  true  body -cavity,  and  the  latter  author  was  forced 
to  the  conclusion  that  Amphioxus  differed  fundamentally 
from  all  the  other  Vertebrates  in  that  the  gill-slits  opened 
into  the  peritoneal  cavity.  Although  that  condition  of 
things  was  hard  to  imagine,  yet  it  seemed  to  be  obviously 
the  case,  since  the  reproductive  organs  appeared  to  lie  in 
the  same  cavity,  and  it  went  without  saying  that  a  cavity 
containing  the  gonads  could  only  be  the  peritoneal  cavity. 
In  reality,  the  gonads  do  not  lie  in  this  cavity ;  they  only 
project  into  it  and  lie  in  a  space  of  their  own  which  is 
separated  from  the  atrial  cavity  by  a  double-layered  mem- 
brane.    (Cf.  Fig.  2.) 

HuxLEV  threw  some  light  on  the  matter  in  1874,  when 
he  compared  the  atrial  or  peripharyngeal  cavity  of  Amphi- 
oxus to  the  opercular  cavity  which  surrounds  the  gills  of 
the  tadpoles  of  the  frog  and  tailless  Amphibia  generally. 
In  the  case  of  the  tadpole,  as  is  well  known,  there  are  some 
four  pairs  of  gill-slits  which  open  at  first  directly  to  the 
exterior.  Subsequently  an  opercular  fold  grows  backwards 
over  them  as  in  fishes,  but  with  this  difference,  that  in  the 


b> 

slil 

m 


IN I'EKNA I.   A. v.  I  TOM  \ '. 


23 


ln)<;-ta(lpolc  the  fold  of  one  side  becomes  continuous  ven- 
trally  with  that  of  the  other,  so  that  in  effect  vvc  have  one 
hiri;"e  semicircuhir  fold  covering  over  the  i^ill-slits.  ICvent- 
u:dly  the  hinder  free  margin  of  the  fold  undergoes  con- 
crescence with  the  body-wall,  so  that  a  single  peribranchial 
cavity  is  formed  about  the  gills.  This  cavity  is  closed  all 
round  except  at  one  point,  usually  on  the  left  side,  but 
sometimes  in  the  mid-ventral  line,  where  it  remains  open 
as  \.\\^  poms  hyancliialis,  or  so-called  spiraculum. 

This  comparison  of  Hux- 
ley's was  extremely  well 
taken,  and  although  the  two 
cavities,  namely,  the  peri- 
branchial cavity  of  the  frog- 
larva  and  the  atrial  chamber 
of  Amphioxus,  are  probably 
by  no  means  homologous,  or 
genetically  related  to  each 
other,  still  the  close  analogy 
that  exists  between  them  is 
most  instructive,  and  yet, 
singular  to   say,   it  did  not 

lead     Huxley     to     a     correct         Pig.  s— Tadpole  of  Froir(A'«;;<2  r/<j- 

interpretation  of   the  atrial  ""'An  n-om  ventral  side.    (OriRinai.) 

cl.  Di'xtrally  placed  cloaca!  aperture, 
chamber.^  m.  Mouth,     sp.  spiraculum;   the   dotted 


Its    true    nature    was    at 


line  indicates  the  extent  of  the  opercular 


chamber,    t.  Tail. 

length  established  by  Rolph 

in  1876.  By  comparing  his  own  observations  on  the  adult 
with  those  of  Kowalevsky  on  the  larva,  Rolph  came  to  the 
conclusion  that  the  atrial  cavity  of  Amphioxus  originated 
by  the  growth  of  two  folds  of  the  body-wall  over  the  gill- 
slits  on  each  side,  and  by  their  subsequent  fusion  in  the 
mid-ventral  line  except  at  one  point,  which  remained  open 


24 


AXATOA/y  or  .iArr/ffoxrs. 


as  the  atrioporo.  Altluni,i;h  the  details  hi  the  formation  of 
the  atrium  are  not  exactly  such  as  they  were  sujiposed  to 
be  by  Rolph  (see  below),  yet  the  end-result  i.:  virtually  the 
same,  and  his  work  marks  a  distinct  advance  in  our  knowl- 
edge of  the  structure  of  Am|)hioxus,  by  showin;^  that  the-^\ 
epithelium  lininj^  the  walls  of  the  atrial  chamber  is  not 
peritoneal,  but  is  derived  by  a  process  of  in-foldin.ij,  from 
the  ectodermic  covering  of  the  surface  of  the  body.  In 
other  words,  the  atrial  cavity,  like  the  ojjercular  cavity  of 
the  Amphibian  tadpole,  is  lined  by  ectoderm. 


i  isci'ra. 

A  bird's-eye  view  of  the  internal  organs,  as  exposed  by 
cutting  the  animal  open  ventrally  by  incisions  extending 
forwards  and  backwards  from  the  atriopore,  is  shown  in 
Fig.  9.  First  and  foremost,  our  attention  is  arrested  by 
the  relatively  enormous  pharynx  occupying  more  than  half 
the  length  of  the  body,  with  its  right  and  left  perforated 
walls  and  parallel  gill-bars  abutting  at  the  mid-ventral  line 
on  the  cndostylc. 

The  alimentary  canal  is  seen  in  the  dissection  to  have  a 
perfectly  straight  course  between  mouth  and  anus,  with 
no  windings  whatever.  Growing  out  ventrally  from  what 
may  be  termed  the  pyloric  region  of  the  intestine,  a  short 
distance  behind  the  pharynx  and  in  front  of  the  atriopore, 
there  is  a  large  diverticulum  ending  blindly  in  front,  which 
in  the  adult  lies  for  the  greater  part  of  its  extent  applied 
against  the  right  wall  of  the  ptiarynx  (Fig.  9).  This  is 
the  so-called  hepatic  ccccmn,  corresponding  to  the  liver  of 
higher  forms.  The  permanent  condition  of  the  liver  in 
Amphioxus  is  comparable  to  its  embryonic  condition  in  the 
Vertebrates,,  where  it  attains  a  much  more  complicated 
structure  in  the  older  stages  by  subsequent  branching  and 


f 

< 

( 

■5 

1 

i 

a 

'!* 

0 

\ 

s 

IS 


tui 


IX  TF.KXAL   ANA  TOM ) '. 


25 


by 


anastomosing  of  the  branch- 
es, etc.  It  is  essentially  a 
iiieilian  ventral  out,i;ro\vth 
from  the  intestine,  and  its 
Iviiii;-  on  one  side  of  the 
pharynx  in  Amphioxus  is 
only  a  secondary  topographi- 
cal necessity.* 

Attached  to  the  lateral 
muscular  body-wall  on  each 
side  are  the  gonadic  pouches, 
which  project  into  the  cavity 
of  the  atrium.  (Cf.  Fi<;.  2.) 
Their  number,  which  is  usu- 
ally twenty-six  pairs,  varies 
slij^ditly,  and  sometimes  there 
are  more  on  one  side  than 
on  the  other,  as  in  Fig.  9. 

The  atrial  cavity  does  not 
end  at  the  atriopore,  but  is 
continued  beyond  it  as  a 
blind  sac  lying  to  the  right 
of  the  intestine,  and  reach- 
ing back  nearly  as  far  as  the  ^ig.  9.— Amphioxus   dissected  from 

.   .  the  ventral  side.    (After  Katukk.  slightly 

anus.    In  rig.  9  the  position   altered.) 

of  this  post-atrioporal  cxtcn-     .  "':  •^""•-'^"^■^'o  "io»tf>  ^^ith  the  huceai 

■^  '  cirri  lying  over  it.    /.  Pharyn.x.    i:  Lndo- 

S!(>)1   of    the    atrium    is    indi-    style.      /.  Hepatic  caecum.     ,<'.  Cionadic 

.  .  r  1  1    pouches,     af.  Position  of  atrioport;;    the 

Cated  by  means    01    a    dotted    post-atrioporal  extension  of  the  atrium  is 

]j.-,g  indicated  by  the  dotted  line  passing  over 

to  the  right  side  of  ;,  the  intestine,    an. 

Finally,  in  Fig.  9,  the  anus  Anus. 

N  H  - 

is  seen  lying  to  the  left  of  stomach. 


\^ 


N   ' 


p. 


■  Note  absence  of  differentiated 


*  The  crocum  is  held  in  position  by  cord-like  attachments  to  the  ligamen- 
tum  denticulatum. 


26 


AXATOMY  OF  AMPIHOXUS. 


the  caudal  fin,  and  the  right  margin  of  the  oral  hood  is 
shown  to  be  continued  round  the  front  end  of  the  body 
into  the  cephalic  expansion  of  the  dorsal  fin. 


Ccelom. 

The  question  now  arises :  if  the  atrial  cavity  is  not 
the  true  body -cavity,  what  has  become  of  the  latter?  In 
order  to  determine  this  point,  it  is  necessary  to  have 
recourse  to  transverse  sections  through  the  body,  such  as 
the  one  represented  in  Fig.  2,  which  is  taken  through  the 
middle  of  the  pharyngeal  region.  In  a  section  like  this, 
the  work  of  tracing  the  limits  of  the  atrial  cavity  is  often 
greatly  facilitated  by  the  presence  of  a  rich  brown  pigment 
in  the  epithelium  lining  its  walls.  We  find,  accordingly, 
that  the  atrial  cavity  has  extended  itself  at  the  expense  of 
the  ccelom,  and  has  reduced  the  latter,  in  the  main,  to  a 
small  space  on  either  side  of  the  dorsal  aorta,  the  aorta 
being  double  in  this  region  (Fig.  2).  This  portion  of 
the  coelom  is  sometimes  spoken  of  as  the  supm-pJioyugeril 
ccclojii,  and  sometimes  as  the  subcJiordal  ccelom,  since  it  lies 
dorsal  to  the  pharynx  on  the  one  hand,  and  below  the  noto- 
chord  on  the  other.  Other  fragments,  so  to  speak,  of  the 
coelom  are  found  accompanying  some  of  the  branchial  bars, 
namely,  every  alternate  one ;  and  another  portion  occurs 
below  the  endostyle.  (See  Fig.  13.)  The  hepatic  caecum 
is  also  surrounded  by  a  division  of  the  coelom,  but  its 
cavity  is  '-.duced  to  a  minimum,  and  the  same  applies  to 
the  C(elom  surrounding  the  intestine  immediately  behind 
the  jjharynx.  Ik'hind  the  atriopore,  as  we  have  seen,  the 
atrial  cavity  is  confined  to  the  right  side,  so  that  on  the 
left  side  of  the  intestine  in  this  region  the  coulom  presents 
its  original  proportions. 


^9 


IN  TERA'A  L   AX  A  TOM  Y. 


27 


■al  hood  is 
:  the  body 


'ity   is   not 
atter  ?     In 
•y  to   have 
ly,  such  as 
rirough  the 
1  like  this, 
ity  is  often 
vn  pigment 
ccordingly, 
expense  of 
main,  to  a 
,  the  aorta 
portion  of 
■pJiaryngeal 
;ince  it  lies 
,v  the  noto- 
jeak,  of  the 
chial  bars, 
ion  occurs 
tic  coecuni 
Ti,   but  its 
applies  to 
bly  behind 
seen,  the 
fiat  on  the 
presents 


Structure  of  Pharynx. 

We  have  already  had  occasion  to  mention  the  fact  that 
the  wall  of  '"he  pharynx  on  each  side  is  perforated  by  a 
great  number  of  vertically  elongated  slit-like  apertures  — 
the  gjll-clefts.  In  the  middle  region  of  the  pharynx  the 
"ill-slits  stretch  almost  from  the  roof  to  the  base  of  the 
jiharynx,  but  in  front  and  behind  they  gradually  become 
much  lower  in  vertical    height  (Fig.    10).      In  the    fully 


C  C.C 

Fig.  10. -Anterior  jjoriion  ol  rigiit  wall  of  jiliarynx,  to  show  arrangement  of 
skeliMal  ro>ls.     (After  j.  Mi' I. IKK.) 

i\  luuiostyle.  e.c.  iMidostylar  ccTlom.  /./;.  Skeletal  rod  of  primary  gill-bar. 
t.b.  .'skeletal  rod  of  tongue-bar,     sy.  Cross-bars  or  syiiapticula. 

N.i').  —  A  simple  gill-slit  tindividcil  by  a  tongue-bar  shoukl  have  been  inserted 
in  the  tigme  in  front  of  the  first  double  slit.      |.  Midler  failed  to  observe  this. 

expanded  condition  the  gill-slits  are  nearly  vertical,  as  in 
l"'ig.  10,  but  by  the  contraction  of  the  transverse  muscles, 
which  lie  in  the  floor  of  the  atrium,  they  are  often  found 
to  be  directed  verv  obliquelv  backwards,  and  this  is  the 
condition  in  which  tliey  almost  invariably  occur  in  pre- 
served specimens.  That  is  the  reason  why  so  many  (»f 
the  bars  are  involved  in  a  single  transverse  section.  (Cf. 
Fig.  2.)     On  account  of  the  prodigious  extent  to  which 


J 


28 


ANA  TO  J/ V  OF  AMPIIIOXUS. 


\\\ 


%. 


the  pharynx  is  perforated  by  the  gill-clefts,  it  is  necessary 
for  it  to  have  some  sort  of  skeletal  support  to  prevent  it 
from  collapsing.  This  is  effected  by  a  series  of  stiff  gelat- 
inous rods  which  lie  in  the  walls  bounding  the  gill-clefts. 
These  rods  have  the  consistency  of  chitin,  —  the  material 
that  forms  the  exoskeleton  of  insects,  —  and  are  insoluble 
in  caustic  potash.  The  portion  of  the  pharyngeal  wall 
which  lies  between  any  two  gill-slits  is  called  a  gill-bar. 

It  will  be  seen  at  once  in  Fig.  lo  that  there  are  two 
kinds  of  skeletal  rods  differing  in  the  behaviour  of  their 
lower  extremities.  Dorsally  the  rods  arch  over  into  one 
another,  but  vf  itrally  they  are  independent,  and  every 
alternate  rod  is  bifurcated,  while  the  somewhat  shorter 
intermediate  rods  end  plainly.  The  forked  rods  form  the 
skeletal  support  of  the  primary  gill-bars,  while  the  inter- 
mediate simple  rods  support  the  secondary  gill-bars,  or 
tougue-bars,  as  they  are  usually  called.  The  primary  bars 
constitute  the  walls  of  the  primary  gill-clefts.  The  latter, 
at  their  first  origin,  appear  as  simple  oval  openings  in 
the  wall  of  the  pharynx.  Later  on  the  simple  opening 
becomes  divided  into  two  by  the  gradual  dipping  down- 
wards of  its  dorsal  margin  until  it  meets  and  fuses  with 
the  ventral  margin.  In  this  way  is  the  tongue-bar  formed 
and  the  gill-slit  doubled.  (Cf.  Fig.  ii.)  The  statement 
which  was  made  above,  therefore,  that  there  could  be  as 
many  as  iSo  openings  on  each  side  of  the  pharynx,  signified 
that  there  might  be  some  ninety  pairs  of  primary  gill-clefts. 

Eventually  the  gill-slits  become  still  further  subdivided, 
though  not  so  obviously,  by  the  formation  of  small  cross- 
bars which  pass  over  from  one  primary  bar  to  another, 
skipping  over  the  tongue-bar,  although  eventually  fusing 
with  the  skeletal  axis  of  the  latter  on  their  inner  faces 
r  ig.  lo). 


m 


anl 

fu[ 

Wj 


^ 


ssary 
ent  it 
cjclat- 
:lefts. 
itcrial 

I     Willi 

Ul-bar. 

-o  two 

{  their 

to  one 
every 

shorter 

rm  the 

2  inter- 

mrsy  or 

,ry  bars 

-  latter, 
ings  in 

opening 

r  down- 

jes  with 

formed 

atement 

Id  be  as 
signified 
ill-clefts, 
bdivided, 
all  cross- 
another, 
ly  fusing 
ner  faces 


IN  TERXA  L   AXA  7  OM  Y. 


29 


Evolution  of  the  T/iyf/iiis  Gland. 

Tongue-bars,  like  those  occurring  in  the  gill-slits  of 
Amphioxus,  are  only  known  otherwise  to  occur  in  the 
remarkable  worm-like  creature,  Balntioglossits.  In  the 
higher  Vertebrates  they  appear  to  be  entirely  absent,  but 
in  the  course  of  the  development  of  the  higher  forms 
there  is  a  structure  which  arises  from  the  dorsal  wall 
of  the  gill-slits  which  may  very  well  be  the  homologue 
<.)f  the  tongue-bars  of  Amphioxus.     This  structure  is  the 


ol£       vel     pb-b    nph     c^i        i^t 


Fig.  IT.- — Anterior  region  of  younc;  Amphioxus  from  left  side.  (After  W'll.I.KY  ; 
tlio  renal  tubules  inserted  after  I'>in'i;KI.) 

(//.  Atrium,  ci.  Buccal  cirri,  c/i.  Notochord.  d.f.  Dorsal  fin-elianibers.  v.  V.yn- 
s]iot.  £•/,'(/.  Endostyle.  /lep.  Outgrowin<j  ccrcum ;  the  index  line  passes  through 
one  of  |.  Miiller's  renal  pa])illa'.  iih-t.  Metapleural  fold.  /////.  Nejihridia  or  renal 
tubules,  tit.  Spinal  cord.  olf.  Olfactory  pit.  p/i.b.  I'eripharyngeal  ciliated  band. 
tb.  'ronguc-bars.     vel.  Velum. 

tliynius  inland.  The  thymus  is  one  of  those  enigmatical 
ductless  glands  which  are  so  eminently  characteristic  of 
the  Vertebrate  organisation,  and  are  of  the  utmost  jjhys- 
iological  and  pathological  importance  to  the  individual. 
Ill  their  structure  and  development  they  give  clear  indi- 
cations of  having  undergone  an  extensive  change  of 
function  in  the  course  of  their  evolution. 

The  thymus,  therefore,  is  presumably  the  derivative  of 
an  ancestral  organ,  which  formerly  possessed  an  active 
function  as  opposed  to  the  apparently  passive  function 
which  this  gland,  and  others  like  it,  exercise  in  che  exist- 


i^,>' 


ff. 


-'Xi, 


ti; 


30 


AA'AI'OMV   OF  AMPH/OXi'S. 


Hm 


1^: 


\  :i 


ing  Craniota.  Amphioxus  has  hitherto  been  regarded 
as  forming  a  marked  exception  among  the  Vertebrates 
in  having  no  thymus,  whereas  one  might  reasonably  have 
expected  to  find  here  the  representative  of  the  thymus 
in  full  activity.  Although  contrary  to  the  nrevailing 
impression,  I  would  suggest  that  the  thymus  is  repre- 
sented in  Amphioxus  by  the  very  actively  functional 
tongue-bars. 

DoHRX  has  shown  that  in  the  Selachian  (shark) 
embryo  the  thymus  arises  by  a  series  of  distinct  cell- 
proliferations  from  the  epitheliimi  of  the  dorsal  wall   of 

the  successive  gill-slits  with 
the  exception  of  the  first, 
which  is  the  .^,pu-acle  (Fig. 
12).  Sometimes  these  pro- 
liferations cause  a  small  pro- 
jection downwards  into  the 
gill-slit,  comparable  to  an 
incipient  tongue-bar.  Event- 
ually these  separate  thymus 
rudiments  pass  inwards  and 
come  together  so  as  to  form 
the  definite  thymus  gland. 
Dohrn    concluded   from    its 


cai^ 


uav 


JV 


Fig.  12. —  Horizontal  section  through    modc     of      Origin      that      the 
the   branchial    region    of  an    embrvo   of    .i  i^.     i     r  ^i 

.So'///«w  t,;///V«/a  to  show  the  rudiments    tnymUS      rCSUltCCl    irom      tllC 

of  the  thymus.    (After  DoiiRN.)  mctamorphosis    and    intro- 

sp.  Spiracle,     cav.  Cavity  (cuelom)  of 

branchial  bar.     I,  II,  III.  I-'irst,  second,  VCrsion  of  gill-filamcntS  ;  and 

and   third   irill-pouches.     jusr.v.    lutrular  •  •    i.     r  r      ^    ^i  •        ■  r 

vein.    thy.  Thymus  rudiments.  1"  POl"^  of  fact,  thlS  View  ot 

its  morphological  nature  is 
probably  correct.  But  the  tongue-bars  of  Amphioxus, 
which  correspond  closely  in  position  to  the  thymus  rudi- 
ments in  the  Selachian  embryo,  and  are,  like  the  latter, 


■••^ 


INTERNAL  ANATOMY. 


31 


;  (yarded 
ebrates 
ly  have 
thymus 
e  vailing 
;  rcpre- 
nctional 

(shark) 
ict    cell- 
wall   of 
lits  with 
he  first, 
jle  (Fig. 
lesc  pro- 
mall  pro- 
into  the 
le   to    an 
Event- 
thymus 
ards  and 
to  form 
gland, 
rom    its 
tiat    the 
om    the 
intro- 
nts ;  and 
s  view  of 
nature  is 
iphioxus, 
nus  rudi- 
le  latter, 


essentially  epithelial  structures,  are  nt)thing  else  than  gill- 
filaments  or  gill-lamellx.  It  appears,  therefore,  that  we 
are  justified  in  supposing  that  the  tongue-bars  of  Amphi- 
oxus  are  the  functionally  active  organs,  of  which  the  thymus 
of  the  higher  forms  is  a  metamorphosed  derivative. 

Etidostyle. 

Returning,  then,  to  the  consideration  of  the  more  inti- 
mate structure  of  the  pharynx,  —  the  endostyle  has  been 
already  mentioned  as  a  ven- 
tral groove  of  the  pharynx 
accompanying  the  latter 
throughout  its  whole  length. 
A  transverse  L.ection  of  it 
alone  is  shown  in  Fig.  13. 
It  is  composed  of  very  high 
columnar  cells  arranged 
throughout  in  one  layer,  al- 
though the  tenuity  of  the 
cells,  whose  nuclei  are  often 
placed    at    different    levels, 

^      ,1       •  ■  Fie.  13.  —  Transverse  section  through 

gives  rise  to  the  impression  ,„,i,,t,,,^„  Ampi>ioxus.  (Af.e.-  lA 
of  cells  occurring  in  several   kkstk.r,  siiRhtiy  altered.) 

e.a.  Branchial  artery  with  hlood-clot. 
layers.       The  four  groups   of    ^.c.  Endostylar  coelom.    U-.  Skeletal  plate. 

gland-cells,  placed  symmet- 
rically two  on  either  side  of  the  median  line,  are  the 
distinguishing  feature  of  the  endostyle.  The  cells  are 
all  ciliated,  but  those  in  the  middle  line  bear  a  bunch  ol 
specially  long  cilia,  which  are  of  great  importance  in 
putting  in  motion  the  cord  of  mucus  secreted  by  the 
glandular  cells  of  the  endostyle.  Below  the  endostyle,  there 
is  a  well-defined  portion  of  the  true  body-cavity  in  which 
the  branchial  artery  lies.     This  is  the  cudostylar  avloiii. 


e.a 


,^?ai 


i. 


m 


32 


ANATOMY  OF  AMPIIIOXUS. 


Besides  the  rods  in  the  yill-bars,  there  is  .  series  of 
paired  skeletal  plates  lying  immediately  below  the  entlo- 
stylar  epithelium  (Fig.  13).  These  plates  correspond  in 
number  to  the  primary  gill-slits.  Their  shape  and  arrange- 
ment are  shown  in  Fig.    14.     They  slightly  overlap  eacli 

other,  and  alternate 
with  one  another  just 
as  the  primary  gill-slits 
alternate.  This  alter- 
nation of  paired  struc- 
tures is  of  very  general 
occurrence  in  Amphi- 
oxus,  and  affects  almost 
every  system  of  organs, 
—  such    as    muscular, 

Fig.  14.  -  Loner  portions  of  skeletal  rods  of  nerVOUS,  reproductive, 
pharynx  with  three  pairs   of  endostylar  plates, 

seen  from  above.    (After  si'K.NCKu) '  and  branchial  systcms. 

The  substanee  of  the  skeletal  rods  passes  into  j  V->         f    t-     1 

that  of  the  endostylar  plates  (^./>).  thus  producing  ^^    maybe    Stated    aS    a 

an  arcade  like  the  cover  of  a  shoe  (Spengel).  rrencral    rulc    tO    which 

sy.  Cross-bars  (synapticula).  ^ 

there  are  some  excep- 
tions, that  with  regard  to  the  paired  organs  of  Amphioxus, 
the  organs  of  one  side  {e.g.  myotomes,  primary  gill-slits, 
gonads,  spinal  nerves)  do  not  lie  opposite  to  their  antimens 
on  the  other  side,  but  alternate  vith  them. 

BrancJiial  Ban. 

The  structure  of  the  branchial  bars  is  shown  in  section 
in  Fig.  15.  Both  kinds  of  bars,  primary  and  secondary, 
have  the  same  general  appearance,  being  compressed  and 
band-like,  but  the  secondary  bar  is  the  smaller  of  the  two. 

The  chief  point  of  difference  between  them  is,  that  in 
the  primary  bar  a  portion  of  the  coelom  is  involved,  which 
is  absent  in  the  secondary  bar.     In  the  case  of  the  primary 


IXTERXAL  AX  A  TOMY. 


33 


series  ot 
the  einlo- 
t^spond  in 
d  arran^i^e- 
jdap  each 

alternate 
other  just 
11-shts 


rygi 
rhis  alter- 
Lired  struc- 
iry  general 
in  Amphi- 
eets  almost 
n  of  organs. 

muscular, 
productive, 
.al  systems, 

tated  as  a 
to  which 
ome  excep- 
mphioxus, 

y   gill-slits, 

r  antimcns 


in  section 

secondary, 
iressed  and 
)f  the  two. 

is,  that  in 
Ived,  which 

he  primary 


bar  (iMg.  15  /')»  commencing  from  the  outside,  that  is  to 
sa\-,  from  the  c([\;ii  turned  towards  the  atrial  cavity,  we 
have  first  a  patch  of  columnar  atrial  epithelium,  at  the  cor- 
ners of  which  some  of  the  cells  contain  a  quantity  of  the 
rich  brown  pigment  which  has  been  referred  to  above  as 
being   characteristic    of    the   atrial    epithelium    generally. 


AD  c 

Fig.  15,  .-/  and  11.  —  Transverse  sections  through  primary  (B)  and  secondary 
(.■;i  i,'iH-bars.     (After  HK.NHAM,  slightly  altered.) 

ii.i-.  Atrial  epithelium,  b.e.  15ranchial  epithelium,  c.  C(rloniie  space  of  jirimarv 
bar.  si:.  Skeletal  rods.  v.  Ca-lomic  vessel  of  primary  bar.  v" .  External  vessel 
of  li'iiii  bars.     v'".  Internal  vessel  of  both  bars. 

N.H.  —  Henham  holds  the  space  at  the  inner  edge  of  the  skeletal  rod  of 
tonuui'-bar  for  a  blood-vessel. 

C.   Isolated  ciliated  cells  of  the  branchial  epithelium.      (After  Lanc;kku.\ns.) 

Next  comes  a  cavity  which  is  a  portion  of  the  coelom,  and 
is  lined  by  the  flat  coelomic  epithelium.  In  fact,  the  dor- 
sal, or  subchordal  coelom  on  each  side  (cf.  Fig.  2)  is  put 
in  connection  with  the  endostylar  coelom  by  a  canalicular 
detachment  of  the  ccelom  which  accomi 


ipani 


primary 


34 


ANATOMY   OF  AMP/flOXUS. 


bar  of  the  pharynx.  Wedged  in  between  the  ccelomie  and 
atrial  epithelia  of  the  primary  bar  is  a  small  blood-vessel, 
V.  Internal  to  the  cadomic  space  lies  the  skeletal  rod, 
which  in  section  has  the  shape  of  a  triangle,  at  whose  apex 
there  is  another  blood-vessel,  v".  The  sides  and  inner 
edge  of  the  bar  are  composed  of  the  ciliated  pharyngeal 
epithelium.  The  cells  of  the  latter  ap  always  arranged  in 
a  single  layer,  but  at  the  sides  of  the  gill-bars  they  are 
very  long  and  thin,  and  the  nuclei  are  crowded  together  at 
different  layers  so  as  to  give  the  idea  of  a  many-layered 
epithelium  (Fig.  15  C).  The  cells  of  one  side  of  the  bar 
are  in  juxtaposition  with  those  of  the  opposite  side,  except 
at  a  point  near  the  internal  C(\gc  of  the  bar,  where  a  space 
occurs.     In  this  space  there  is  a  third  blood-vessel,  v"'S' 

In  the  secondary  bar,  there  is  no  vessel  corresponding 
to  the  one  marked  v  in  the  primary  bar,  and  the  vessel 
that  corresponds  with  v"  is  entirely  enclosed  within  the 
skeletal  rod. 

The  dorsal  wall  of  the  pharynx  is  closely  appressed 
against  the  sheath  of  the  notochord,  and  separates  the  two 
dorsal  aortac  from  one  another.  It  has  here  the  form  of  a 
groove  running  parallel  with  and  opposite  to  the  endostyle. 
It  is  known  as  the  JiyperbmncJiial  groove.  (Cf.  Fig.  2.) 
Two  special  tracts  of  ciliated  epithelium  form  the  sides  of 
it,  and  pass  downwards  in  front  to  join  the  anterior  extrem- 
ity of  the  endostyle  on  each  side.  In  front,  where  these 
tracts  bend  downwards  with  a  crescentic  curve,  they  are 
known  2i?,  X.\iQ  pcrip/imyugcal  bands.     (See  Fig.  ii,ph.b.) 


1  1  ■! 


Musailatnrc. 

The  musculature  of  Amphioxus  is  composed  almost 
entirely  of  striated  muscle-fibres.  Involuntary  or  smooth 
muscle-fibres  are  remarkable  for  their  extreme  tenuity,  and 


IXTKKA'AI.   AXA  T(  )M  Y. 


35 


lie  and 
-vessel, 
al  rod, 
sc  apex 
1  inner 
ryngeal 
n<;ed  in 
hey  are 
ether  at 
-layered 
the  bar 
:,  exeept 
;  a  space 
,  x;"'.« 
iponding 
le  vessel 
thin  the 

ppressed 
J  the  two 
orm  of  a 
ndostyle. 
Fig.  2.) 
sides  of 
extrem- 
ere  these 
they  arc 
ph.b) 


almost 
)r  smooth 
uity,  and 


in  correlation  with  this  condition  is  to  be  noted  the  absence 
ot"  a  distinct  synipat/ictic  ucrvoits  system. 

The  striated  muscles  can  be  arranged  in  two  groups  : 
(i.)  the  parietal  muscles  constituting  the  myotomes  or 
muscular  segments  of  the  body,  and  (ii.)  the  visctral 
muscles  which  arise  independently  of  the  myotomes  and 
are  not  segmentaliy  arranged.  The  smooth  muscle-fibres, 
which  occur  on  the  walls  of  the  alimentary  canal  and 
l)l()()d-vessels,  may  be  grouped  together  as  the  splaticlinic 
muscles. 

The  parietal  muscles  are  the  great  longitudinal  muscles 
which  make  up  the  thick  lateral  walls  of  the  body.  In 
Amphioxus  they  form  collectively  the  essential  organ  of 
locomotion.  The  portion  of  them  lying  next  to  the  atrium 
on  each  side,  and  stretching  from  the  notochord  to  the 
base  of  the  myotome,  is  placed  at  an  angle  to  the  rest,  and 
has  a  more  vertical  direction.  (Cf.  Fig.  2.)  This  has 
been  described  by  Schneider  as  the  rectus  abdominis.  It 
probably  co-operates  with  the  muscles  of  the  floor  of  the 
atrium  to  cause  the  contraction  of  the  latter  cavity  for  the 
purpose  either  of  expelling  water  or  reproductive  elements 
through  the  atriopore. 

The  visceral  muscles  consist  of  (a)  the  transverse  muscles 
stretching  across  the  floor  of  the  atrium  (cf.  Fig.  2),  (/3) 
muscles  of  the  oral  hood  and  cirri,  (7)  sphincter  muscle  of 
the  velum  (cf.  Fig.  7),  (8)  anal  sphincter. 

All  the  striated  muscles  of  Amphioxus  are  composed 
of  highly  characteristic  flat  lamelliform  plates,  which  can 
often  be  resolved  into  a  great  number  of  finer  fibrils.  In 
the  longitudinal  muscles  of  the  adult,  nuclei  are  very 
rarely  met  with,  but  in  other  places  they  arc  to  be  found  ; 
as.  for  instance,  in  the  fibres  composing  the  velar  sphincter 
(Fig.  16). 


i>  - 


LIBRARY 

NATIONAL  MUSEUM 

OF  CANADA 


3^^ 


AA.r/'O.UV   OF  AMPHIOXUS. 


Mill-! 


This  peculiar  plate-like  muscular  tissue  is  found  in 
connection  with  the  lateral  muscles  only  of  the  Cyclostonic 
fishes.  The  muscle-fibres  of  the  mouth 
and  velum,  as  LAXciKRHAXs  pointed  out, 
closely  resemble  those  found  in  the  walls 
of  the  heart  of  the  hijjjher  Vertebrates. 
In  transverse  section  the  cut  edges  of 
the  longitudinal  muscle-plates  are  to  be 
seen  stretching  across  the  myotome. 
(Cf.  Figs.  2,  26.) 

The  transverse  or  .J7//;-^r/;7V?/ muscles  arc 
divided  by  a  median  longitudinal  septum 
of  connected  tissue  into  right  and  left 
halves.  They  are  further  subdivided  into 
a  series  of  compartments  by  thin  trans- 
verse septa.  These  compartments,  how- 
ever, are  not  arranged  segmentallv,  since 

Fig.  16.-  Isolated  »  f^  ^' 

muscle-fibre    of    the  they  are  morc  numerous  than  the  myo- 

velar  siihincter.   (After    ^  t-.,  ■,        ,    ,  r    x  1 

L.vNCKRHANs.)  tomcs.     Thc  muscle-plates  of  these  mus- 

cles are  placed  edge  on,  so  that  they  do 
not  lie  one  over  the  other  as  the  plates  of  the  myotomes 
tlo,  but  one  behind  the  other.  They  are  attached  to  the 
septum  at  the  base  of  the  myotomes  on  the  one  hand,  and 
to  the  median  septum  or  raphe  on  the  other,  and  also  the\- 
are  attached  at  numerous  points  to  the  connective-tissue 
sheath  or  fascia  which  covers  them  above  and  below. 
When  they  contract,  therefore,  the  floor  of  the  atrium  is 
thrown  into  a  number  of  characteristic  pleats.  (Cf. 
Fig.  2.)  The  individual  muscle-plates  of  Amphioxus  ap- 
pear universally  to  be  devoid  of  a  protecting  sheath  or 
sarcolcvima.  The  sub-atrial  muscles  end  at  the  atrioporc 
round  which  they  form  a  sphincter  muscle. 

The  muscles  of  the  oral  hood,  which  serve  for  the  ercc- 


""% 


INTERiXAL   AXA  TOMV. 


37 


ound  in 
clostonic 
e  mouth 
itod  out, 
the  wall^- 
•tcbrates. 
edges  of 
ire  to  be 
nyotomc. 

uscles  arc 
il  septum 
and   left 
i-ided  into 
bin  trans- 
3nts,  how- 
ally,  since 
the  myo- 
hese  mus- 
\\.  they  d(i 
myotomes 
od  to  the 
land,  and 
also  they 
tive-tissue 
nd  below, 
atrium  is 
fcats.     (Cf. 
iioxus  ap- 
sheath  or 
atriopore, 

r  the  erec- 


tion and  supination  of  the  buccal  cirri,  consist  of  two  por- 
tions, an  inner  and  an  ontcr  {Vvi,.  17).  The  outer  one, 
bv  whose  contraction  the  cirri  are  retracted  in  such  a  way 
that  they  come  to  lie  across  the  entrance  to  the  mouth, 
those  of  one  side  interlacing 
with  those  of  the  other  so 
as  to  form  a  i)erfect  barrier 
to  the  mouth,  is  a  powerful 
muscle  lying  outside  the 
bases  of  the  cirri.  The 
inner  mu.scle,  which  appar- 
ently   serves    to    erect    the 

cirri       consists      of      distinct  Fig.  17. -Muscles  of  the  oral  hood. 

Lim,      LUIl^5lbl^>      in      uisuueu    (After  L.xmikkuans.) 

muscular     tracts      lying      be-  w.^.     outer     muscle     (m.    externus) 

whose    filires   interlace  with  those  of  the 
tween  every  two  consecutive    velar  sphincter  {v.sp).    m.,.  Inner  muscle 
■,.,.:  (m.  internus). 

The  sphincter  muscle  of  the  velum   has  been  already 
referred  to.     (Cf.  Fig.  7.)     A  sphincter  muscle  of  a  simi- 
lar character  also  surrounds 


fnt 


00 


O 


1       if      \ 


-z 

_    ...4. 


the  anus. 

The  septa  which  separate 
the  myotomes  from  one 
another  are  composed  of 
fibrous  connective  tissue. 
The  fibres  are  imbedded  in 

Fig.   18.  —  Diagram    illustrafinc    the  i    i.-  ^    •  t>, 

.hrterent  layers  of  the  integument.    ^After    "^     gelatUlOUS     matrix.         Tho 

liAisciiFK.)  salient  feature  in  connexion 

/.  l-.piclermis.     3.  Outer  laver  of  cutis        •   ,        , 
(liasement  membrane  of   Hatschek   and    With     the     entire     COnnCCtivC 

':;;;;r;:;,i;  "rLin:;;;;"r,:;£  tissue-systo,,,  of  Amphio^us 

5.  Epithelial  layer  of  cutis  (limiting  mem-  is  the  great   preponderance 

brane).  °  *       ' 

of   the  gelatinous    element. 

It  forms  the  bulk  of  the  dorsal  and  ventral  fin-rays,  and 

I  of    the    cephalic    and    caudal    integumentary   expansions. 


'il 


d  -  ■ 


m^  ■• 


38 


ANATOMY   OF  AM/'///O.Vi'S. 


i'l 


ii 


The  middle  layer  of  the  cutis  below  the  epidermis  (et. 
V'Vf^.  18)  is  composed  mainly  of  this  tissue  with  radial 
fibres  superadded.  In  the  metai)leural  folds  it  attains  a 
•i^reater  development  than  in  the  rest  of  the  integument. 
(Cf.  V'lg.  2.)  It  also  constitutes  the  midille  layer  of  the 
sheath  of  the  notochord,  but  the  fibres  in  this  case  run 
concentrically,  and  not  radially.*  The  outermost  layer 
of  the  cutis  (Fig.  18)  and  the  innermost  layer  of  the 
sheath  of  the  notochord  are  composed  of  a  i)eculiar  and 
very  highly  refringent  and  homogeneous  tissue  of  the 
same  order  as  that  which  forms  the  skeletal  rods  of  the 
pharynx.*^  The  layer  of  connective  tissue  which  separates 
the  myotomes  from  the  body-cavity,  and  which  springs 
out  from  the  base  of  the  notochordal  sheath  (Fig.  2),  occu- 
pies the  same  position  as  the  ribs  of  the  higher  Vertebrates. 

NOTES. 

I.  (p.  15.)  Metapli-iiral  Folif^.  —  In  the  development  of  the 
paired  fins  of  Selacliians  it  was  discovered,  in  1876,  by  IUlkoi'r, 
that  at  a  certain  stage  there  appears  along  each  side  of  the  body 
"a  thickened  line  of  e])il)last  {i.e.  ectoderm),  which  from  the  first 
exhibits  two  special  developments."  "These  two  special  thick- 
enings are  the  rudiments  of  the  paired  fins,  which  thus  arise  as 
special  developments  of  a  continuous  ridge  on  each  side,  precisely 
like  the  ridges  of  epiblast  which  form  the  rudiments  of  the 
unpaired  fins."  After  giving  more  details,  Ikilfour  says,  " 'I'lie 
facts  can  only  bear  one  interpretation,  viz.  tluit  the  liinhs  arc  the 
inn  nan  fs  of  continuous  lateral  fins. ^^ 

Shortly  afterwards  (1877),  but  quite  independently,  Jamks  K. 
Thachkr  was  led  by  a  comparative  study  of  the  adult  skeleton  of 

*  Tn  that  portion  of  the  sheath  of  tlie  notochord  which  lies  aln)ve  the  dor- 
sal }:;roove  of  the  ])harynx  thers  is  a  special  tract  of  connective-tissue  fibres 
which  run  Knigitudinally.  .A  similar  tract  can  sometimes  he  ojiserved  in  tlie 
dorsal  portion  of  the  sheath  below  the  nerve-curd.  (Schneider,  l.ankester, 
Spcngel.) 


;,  l^m- 


NOTES. 


39 


Selachians  and  other  fishes,  to  a  belief  in  the  honiudynamy  <>f 
median  ami  paired  tins,  and  lie  therefore  concUided  that  tiie 
1  liter  arose  as  differentiations  from  primitively  continuous  lateral 
fuis  just  as  the  median  fms  are  obviously  differentiated  from  a 
(ontinuous  dorsal  and  ventral  tin-fold.  'I'liaeher  further  sugj^e^ted 
til, It  the  original  (  ontinuous  lateral  tins  were  rejjresented  in  Am- 
phioxus  by  the  iiieta|)leural  folds.  He  said,  "As  the  dorsal  and 
anal  fins  were  specialisations  of  the  median  folds  of  .\mphio.\us, 
so  the  i)aired  fins  were  sijccialisations  of  the  two  lateral  folds 
(ineta|)leural  folds),  which  are  supplementary  to  the  median  in 
completing  the  circuit  cjf  the  body." 

It  has  recently  been  observed  by  Professor  \\.  A.  Axdkkws,  that 
in  a  new  species  of  Amphioxus  from  the  Bahamas,  the  right  meta- 
plcural  fold  is  continued  behind  into  the  median  ventral  fin. 
Subsecjuently  I  found  the  same  condition  to  obtain  in  a  species 
of  Amphioxus  from  the  Torres  Straits. 

I-'rom  these  observations,  and  froii  the  fact  that  the  right  half 
of  the  oral  hood  (which  apparently  arises  in  continuity  with  the 
right  metapleur  —  I'ii/c  infra')  is  continued  in  front  into  the 
cephalic  expansion  of  the  dorsal  fin,  it  would  appear  that  there 
is  a  great  measure  of  truth  in  Thacher's  suggestion,  notwithstanding 
the  fact  that  in  the  condition  in  which  we  find  them  in  the  exist- 
ing Amphioxus,  the  metapleural  folds  do  not  Junction  as  fins. 
Thacher's  hypothesis  has 
also   been   supported   by 


2.   (p.  lO.  )    Iheaccom-  pjg_  iQ._niapmm  illustratint,'  (by  a  dotted 

paining  diagram  (  Kig.  19)    lint")  the  course  ot  the  food  as  it  i)asst>s  tliioiinh 

will  serve  to  illustrate  the 
actual  course  of  the  fooil 
through  the  pharynx  of 
Ani])hioxus,  as  recently 
determined  by  Andrkws, 
from  observations  on 
transparent  specimens  from  the  Bahamas.  The  food,  enveloped 
in  the  mucous  secretion  of  the  endostyle,  iiasses  along  the  dor- 
sal groove  of  the  pharynx  (hyperpharyngeal  groove)  into  the 
intestine. 


the  pharynx  and  intestine  of  Amphioxus.     (After 
Anukkws.) 

Tile  small  diverticulum  on  the  dorsal  side  of 
the  oral  hood  represents  the  groove  of  Hatschek 
somewhat  exagi^erated.  The  arrows  behind  tlie 
cu.cum  inchcate  the  rotation  to  which  the  food  is 
here  subjected  by  the  action  of  tlie  cilia  of  the 
intestinal  epithelium. 


WfWW 


40 


AA'ATOJ/y   OF  AM  PHI  OX  US. 


3.  (p.  10.)  On  those  occasions  on  which  Amphioxus  is  not 
buried  in  the  sand,  but  lies  on  the  surface  of  the  santl,  occasions 
which  freciuently  occur  when  it  is  kept  in  captivity,  and  especially 
after  having  been  confined  for  a  consideiable  length  of  time,  it 
lies  on  one  side,  as  mentioned  in  the  text.  The  percentage  of 
instances  in  which  it  lies  on  the  right  or  left  side  has  not  been 
taken,  and  consequently  it  is  not  possible  to  say  that  it  prefers  lying 
on  one  side  rather  than  on  the  other.  Since  the  olfactory  pit  and 
tiie  anus  occur  on  the  left  side,  it  is  conceivable  that  it  prefers 
to  lie  on  the  right  side.  If  this  had  been  a  ilefuiite  habit,  it 
wouUl  i)robably  not  have  escaped  tiie  observation  of  Johannes 
Miiller.  It  is  a  fact  which  is  too  frecjuently  overlooked,  that  the 
lying  on  one  side  is  entirely  incidental,  anc'  is  emphatically  not 
the  result  of  adaptation  to  a  i)eculiar  mode  of  life,  as  it  is  in  the 
case  of  the  Pleuronectidai. 

4.  (p.  1 1.)  Species  and  Distribution  of  Ampliioxtis.  —  A  useful 
synopsis  of  the  genus  Branchiostoma  has  recently  been  prepared  by 
AxDRKAVs,  as  an  appentlix  to  his  paper  on  the  remarkable  species 
which  occurs  at  the  Bahamas.  In  this  species  there  is  a  long 
caudal  process  into  which  the  notochord  extends.  It  is  an  active 
swimmer,  donadic  jiouches  are  only  i)resent  on  the  right  side, 
those  on  the  left  being  suppressed.  The  latter  is  also  true  of 
Braneliiostoma  eultelliim.  The  peculiarities  of  the  species  from 
the  Bahamas  were  such  that  Andrews  deemed  it  necessary  to  form 
a  new  genus,  Asymmetron. 

In  the  table  of  species  on  page  41  it  will  be  noticed  that  the 
lengths  of  the  different  species  are  not  in  any  proportion  to  the 
number  of  myotomes. 

Insufficiently  described  species  occur  off  the  coasts  of  yapan, 
Ceylon,  and  Fiji  Islands.  It  is  interesting  to  note  that  while  in 
Europe,  Amphioxus  occurs  as  far  north  as  Scandinavia,  on  the 
Atlantic  coast  of  North  America,  Chesapeake  Bay  appears  to  be 
its  northern  limit,  and  it  is  therefore  wholly  unknown  at  the 
Marine  lUological  Station  at  Woods  Holl.  Attention  may  further 
be  called  to  the  simultaneous  occurrence  of  two  distinct  species, 
B.  eultelliim  and  B.  beleheri,  in  the  Torres  Straits.  B.  cultelluni 
is  easily  recognisable  on  account  of  the  unusual  height  of  its  dorsal 
fin. 


/VOTES. 

41 

NVMIIKK 

Length 

Namk  ok  Species. 

OK 

IN    MlI.I.I- 

GeOC.KAI'HICAI.    DiSTRIlUTION. 

Myotomes. 

METKES. 

I    1!   lanccolatum .     . 

59-61 

35-So 

Scandinavia,  Ilclijjoland,  Eng- 
land, I'rance,  Mediterranean, 
and  ChesapeaUe  IJay. 

2.   1!.  caiib.uum      .     . 

58 

43 

15ra/il,  Mouth  of  La  Plata, 
Jamaica,  Tampa  Bay,  ( lulf  of 
Mexico,  Beaufort,  N.C. 

j.  li.  cultellum       .     . 

52-55 

25-35 

Thursday  Island  (Torres 
vStraits),  Moreton  Bay  (E. 
Australia). 

4.  I'l.  liassanum      .     . 

75 

— 

Bass  Straits,  Australia. 

y  11.  liflchcri    .     .     . 

65 

65 

Borneo  and  Torres  Straits 
(Prince  of  Wales  Island). 

6.  H.  flongatum     .     . 

79 

60 

Peru. 

7.   1).  californiense 

69 

70 

San  Diego,  California. 

S.  B.  liicavamim    .     . 

66 

13-16 

Bimini    and    Nassau    Harbour 

=  Asynunctron  lu- 

(Bahamas). 

cayanuui,  Aiulrews 

5.  (p.  22.)  HuxLKY  had  recognised  in  1S74,  in  the  light  of 
Kowalevsky's  work,  that  the  atrial  cavity  of  Amphioxus  was  lined 
by  an  epithelial  layer  derived  from  the  ectoderm,  but  came  to  the 
conclusion  that  it  was,  by  the  very  f;ict  of  its  inversion  within  the 
body,  converted  into  peritoneal  epithelium.  He  applied  the  same 
interpretation  to  the  opercular  chamber  of  the  Amphibian  tadpole, 
and  gave  to  a  body  cavity  of  this  character  the  general  name  of 
t'piiirk.  Rolph's  merit  consisted  in  distinguishing  clearly  between 
atrial  epithelium  and  peritoneal  epithelium,  and  hence  between 
atrial  cavity  and  true  body-cavity. 

6.  (p.  38.)  There  is  a  great  deal  of  difference  of  opinion  as  to 
the  exact  nature  of  that  dense  refringent  tissue  which  forms  the 
outer  layer  of  the  cutis  and  the  skeletal  rods  of  the  gill-bars. 
l.wKi.srKR  regarded  them  both  as  the  products  of  connective 
tissue-cells,  hence  belonging  to  the  mesoderm,  while  H.vrscHKK 
ami  Spkn'gel  looked  upon  the  outer  layer  of  the  cutis  as  the 
]>roduct  of  the  ectoderm,  of  the  nature  of  a  basement  membrane. 
Si'KNGEL  again  has  advocated  the  view  that  the  skeletal  rods  of  the 


.)« 


42 


AX.ITOMY   OF  AMPinOXUS. 


pharynx  are  special  developments  of  the  basement  membrane, 
which  separates  the  two  opposed  epithelial  layers  of  each  gill-bar 
from  one  another.  (Cf.  Fig.  15.)  More  recently,  Benham  has 
described  nuclei  in  the  latter  membrane,  thus  showing  it  to  be  a 
sheet  of  connective  tissue.  In  this  case  the  substance  of  the 
skeletal  rods  should  be  regarded  as  a  variety  of  connective  tissue. 

A  further  difference  of  opinion  prevails  as  to  the  nature  of  the 
space  which  traverses  the  skeletal  rod  of  the  tongue-bar.  Lax- 
KKSTKR  supposed  it  to  be  a  diverticulum  of  the  coelom.  Spem;kl 
and  HovKRi  interpreted  it  as  a  blood-vessel ;  and,  finally,  Benham 
thinks  that  it  is  both,  inasmuch  as  he  conceives  there  to  be  a  blood- 
vessel contained  in  a  cuelomic  space.  It  should  be  added  that 
these  finer  details  are  extremely  difficult  to  determine. 

7.  (]>.  21.)  Lateral  Line.  —  Since  the  lateral  line  constitutes 
one  of  the  most  characteristic  and  constant  features  in  the  organi- 
sation of  fishes,  its  absence  in  Amphioxus  has  always  been  one  of 
the  most  serious  difficulties  in  the  way  of  a  conception  of  this 
animal  as,  in  any  sense,  an  ancestral  form.  It  need  hardly  be 
pointed  out  that  from  whatever  point  of  view  we  regard  Amphi- 
oxus, it  must  necessarily  have  become  specialised  and  modified 
along  its  own  particular  line  of  evolution,  and  cannot,  as  it  stands, 
be  taken  as  a  direct  ancestral  form,  but  rather  as  a  more  or  less 
close  relative  of,  or  an  exceedingly  ancient  offshoot  from,  the 
actual  ancestor  of  the  Vertebrates.  The  modifications  which  it 
has  undergone  will,  as  in  every  other  case,  have  resulted  in  more 
or  less  extensive  changes  both  in  the  function  and  structure  of  dif- 
ferent parts.  Thus,  while  the  metapleura'  folds  are  very  i)r()bably 
the  homologues  of  the  primitive  continuous  lateral  fin-folds,  yet 
in  their  actual  form  and  function  they  may  or  may  not  represent 
the  primordial  condition  of  these  folds.  Certain  peculiar  features 
in  connexion  with  the  origin  and  innervation  of  the  metapleural 
folds  of  Amphioxus  have  led  me  to  form  a  conception  as  to  the 
origin  of  the  lateral  line  sense-organs  which  may  perhaps  have 
some  value  as  a  working  hypothesis. 

In  those  primitive  fishes  which  possessed  the  continuous  lateral 
fin-folds,  it  is  very  clear  that  the  latter  could  not  have  performed 
a  locomotor  fimction,  but  they  must  have  served  jirimarily  as 
balaneers.     Without  going  into  the  difficult  question  as  to  how 


11. )t 
life  ; 
s:!tio| 
n.itol 

lit"  IKI 

||t-'\\| 

III 

l-r  ti 


NOTES. 


43 


such  structures  could  have  arisen  dc  novo,  we  may  at  least  attempt 
to  appreciate  the  necessity  for  their  existence. 

There  is  one  difference  between  the  general  form  of  the  body 
in  Invertebrates  and  Vertebrates  respectively  which  seems  to  be 
of  fundamental  importance,  but  which  has  not  been  sufficiently 
cnijjhasised.  As  a  general  rule,  in  the  Invertebrates,  the  body 
is  not  bilaterally  compressed,  but,  on  the  contrary,  is  either  cylin- 
(hical,  sub-cylindrical,  or  flattened  dorso-ventrally.  Obvious  ex- 
ceptions to  this  rule  are  presented  by  the  Lamellibranchiate 
Molluscs  and  by  many  Arthropods;  but  these  exceptions  are 
readily  intelligible  as  secondary  modifications. 

On  the  other  hand,  in  the  more  primitive  Vertebrates  {i.e. 
fishes),  the  bilateral  compression  of  the  body  is  one  of  the  car- 
dinal features  of  the  external  form.  To  this  fundamental  rule 
there  are  of  course  exceptions  afforded,  for  example,  by  the 
skates ;  but  it  is  a  self-evident  fact  that  these  again  have  arisen 
by  secondary  modification  from  originally  bilaterally  compressed 
forms.  With  the  evolution  of  the  pentadactyle  appendages  and 
the  assumption  of  a  terrestrial  existence,  the  shape  of  the  body 
in  the  higher  Vertebrates  has  undergone  such  changes  that  the 
primitive  bilateral  compression  of  the  body  is,  as  a  rule,  only 
present  at  some  period  of  the  embryonic  development. 

Aniphioxus  exhibits  the  chan.>.teristic  vertebrate  bilateral  com- 
pression of  the  body  in  a  very  tyjiical  m.anner  ;  while  Balano- 
glossus  shows  invertebrate  affinities  in  regard  to  the  shape  of  the 
body,  which  is  sub-cylindrical. 

The  bilateral  com])ression  of  the  primitive  vertebrate  body  did 
not  arise  in  itself  as  a  si)ecial  adaptation  to  a  particular  mode  of 
life  ;  but  rather  in  correlation  with  other  characters  of  the  organi- 
sation. The  develojiment  of  the  dorsal  medullary  tube  and  the 
notochord  above  the  digestive  tube  and  the  concentration  of  the 
myotomes  would  necessarily  lead  to  a  bilaterally  compressed  form 
of  body.  We  see  this  not  only  in  fishes,  but  in  the  course  of  the 
development  of  all  Vertebrates. 

It  is  obvious  that  such  a  shape  of  the  body  is  highly  unfavourable 
tor  the  maintenance  of  the  ecpiilibrium  except  with  the  assistance 
of  some  special  mechanical  and  sensory  apparatus. 

Xow  in  Amphioxus,  the  metapleural  folds,  whatever  their  exact 


5ti 


\4 


'  ■>>: 


44 


ANATOMY  OF  AMPIIIOXUS. 


function  may  be,  do  not  serve  in  any  way  as  balancers ;  and,  as 
mentioned  in  the  text,  Amphioxus  lias  \\^  means  of  maintaining 
its  equilibrium  when  not  actually  swimming. 

We  will  therefore  keep  in  mind  more  especially  those  Palaeo- 
zoic fishes  which  presumably  possessed  continuous  lateral  fin-folds 
serving  as  balancers.  The  nearest  known  fossil  relatives  of  these 
fishes  appear  to  be  the  Cladosciachidic  (see  Hashfokd  Di'.an. 
Contributions  to  the  Morpholoi^y  of  Claiiosclaclic  {C/adottus). 
Jour.  Morph.  IX.  1894.  pp.  87-112.  Also  .\.  Smith  Woodward. 
The  Evolution  of  Fins.     Natural  Science,  I.     1892.     pp.  2S-35). 

The  lateral  fin-folds  may  be  spoken  of  as  mechanical  balancers, 
and  to  render  them  efficient  organs,  there  must  be  a  sensory  appa- 
ratus in  connexion  with  them.  The  suggestion  lies  near  that  the 
ectoderm  which  took  part  in  the  formation  of  the  lateral  fin-folds 
also  produced  the  sense-organs  of  the  lateral  line. 

The  lateral  line,  through  its  capacity  for  receiving  impressions 
of  wave-movements,  etc.,  would  thus  serve  as  the  agent  in  the 
co-ordination  of  such  muscular  activities  as  are  necessary  to  the 
maintenance  of  the  e(iuilibrium. 

Having  been  once  established,  no  special  difficulty  is  presented 
by  the  fact  that  the  lateral  line  has  spread  over  the  head-region. 
Moreover,  it  may  be  taken  as  a  well-established  morphological 
fact  that  the  auditory  organ  (internal  ear)  became  evolved  as  a 
specialisation  of  part  of  the  lateral  line  in  the  cephalic  region,  and 
that  it  therefore  belongs  to  the  same  category  as  the  less  elaborate 
sense-organs  of  the  remainder  of  the  lateral  line. 

As  is  well  known,  the  internal  ear  has  two  functions,  audition 
antl  equilibration.  It  must  be  supposed  that,  at  its  first  origin, 
the  whole  lateral  line  served  in  a  general  way  the  function  of 
equilibration,  and  that  this  function  eventually  became  rhielly 
localised  in  the  semicircular  canals  of  the  ear,  the  remainder  of 
the  lateral  line  perhaps  undergoing  a  slight  change  or  limitation 
of  function. 

It  seems  certain  that  at  first  the  sense-organs  of  the  lateral  line 
must  have  been  innervated  by  spinal  nerves.  This  follows  both 
from  a  //vVp/v"  considerations  and  also  from  the  condition  in  Amphi- 
oxus, where  the  ectoderm  of  the  metapleural  folds  is  innervate  1 
by  the  Rami  eutanei  ventrales  of  the  dorsal  spinal  nerves.     Under 


lat 
w 
re.- 
stoi 


/lavi 

of 

tril 

lirs 

ear 

'liTk 

fi.'i 

!he 


NO  TES. 


45 


md,  as 
taining 

rala^o- 

iii-foUls 

f  these 

Dkan. 

do  (ills). 

)1)\VAR1). 

28-35)- 
ilancers, 

ry  appa- 

that  the 

fin-folds 

pressions 
It  in  the 
•y  to  the 

presented 
x\ -region. 
)hological 
Ived  as  a 
gion,  and 
elaborate 

audition 
St  origin, 

mction  of 
He   rhielly 

Kiinder  of 
limitation 

ateral  line 
lows  both 
in  Amplii- 
innervatoil 
-B.     Under 


these  circumstances  it  is  necessary  to  suppose  with  Eisig  that  the 
l.itrral  line  nerve  {Ramus  lateralis  7'ai^^i)  arose  as  a  collector. 

I'hc  removal  of  the  lateral  line  from  the  immediate  neighbour- 
li(H)il  of  the  paired  fins  in  existing  fishes  is  easily  intelligible  on 
the  uround  that  the  fins  have  become  discontinuous  and  elaborated 
into  effective  locomotor  organs. 

It  is  not  impossible  that  the  lateral  line  nerve  {R.  lateralis  7'ai;i) 
is  lu)niodynamous  with  tlie  remarkable  Ramus  cutaneus  ijuinti 
{ R.  recurreus  trigcmini  ct  facialis  or  Nervus  lateralis  irigemini, 
SiANNRs)  of  Teleosteans,  which  runs  to  the  l)ase  of  all  the  fins, 
paired  as  well  as  unpaired  ;  just  as  the  paired  fins  themselves  are 
known  to  be  homodynamous  with  the  median  fins.  In  this  case 
the  A',  cutaneus  quiuti  would  be  of  primitive  significance,  notwitii- 
standing  the  fact  that  it  is  absent  in  Selachians  ;  and  it  would  be 
another  of  those  features  of  organisation  in  the  possession  of  which 
Teleosteans  exhibit  more  primitive  relations  than  do  the  existing 
Selachians.  (Compare  the  functional  pronephros  of  Teleosteans 
and  the  enUrely  rudimentary  pronephros  of  Selachians.) 

The  above  suggestion  that  the  lateral  line  arose  in  the  first 
instance  as  a  sensory  ecpiilibrating  apparatus  in  conjunction  with 
the  mechanical  equilibrating  ajiparatus  effected  by  the  continuous 
lateral  fin-folds,  will  of  course  meet  with  numberless  difficulties 
when  it  is  attempted  to  carry  it  oat  in  detail.  As  in  some  other 
respects,  so  here,  a  great  difficulty  is  presented  by  the  Cyclo- 
stonies.  It  may,  however,  be  pointed  out  that  if  the  various  con- 
clusions which  have  been  drawn  with  regard  to  the  morphology  of 
Aniphioxus  are  correct,  it  must  be  assumed  that  the  Cyclostomes 
have  entirelv  lost  the  lateral  fin-folds  and  that  the  sense-organs 
of  the  lateral  line  have  secondarily  become  diffused  in  their  dis- 
tribution over  the  body.  The  latter  conclusion  is  also  indicated, 
liistly,  hy  the  fact  that  there  is  a  fiiirly  well  develoj^ed  internal 
ear  in  the  Cyclostomes  which,  as  noted  above,  must  have  been 
differentiated  from  a  ])rimitlve  lateral  line  ;  and  secondlv,  bv  the 
fiet  that  although  the  sense-organs  are  scattered,  there  is  never- 
theless (at  least  in  Petro-nyzon)  a  definite  lateral  line  nerve. 


;,  I 


■■■■  \ 


II. 


ANATOMY   OF    AMPHIOXUS. 


INTERNAL  ANATOMY    {continued ). 


u 


\\A 


I\  the  preceding  chapter  we  have  seen  how  Amphioxus, 
while  possessing  the  general  facies  of  a  fish,  and  the 
primary  essential  attributes  of  a  Vertebrate,  is  nevertheless 
destitute  of  many  of  the  most  obvious  structural  features 
which  we  usually  associate  with  our  conception  of  a  fish. 
Thus  it  has  no  skull,  or,  in  other  words,  it  is  Acnxniate 
(Haeckel).  It  has  no  jaws,  and  is  therefore  a  Cyclostomc, 
as  opposed  to  a  GnatJiostouic.  Finally,  it  has  no  paired 
sense-organs  and  no  paired  muscular  fins.  Its  eye-spot 
is  median,  like  that  of  a  Cyclopean  monster.  There  is  no 
trace  of  an  auditory  organ  of  any  kind,  while  the  single 
so-called  olfactory  pit,  abutting  on  the  anterior  end  of  the 
nerve-tube,  has  been  regarded  as  an  indication  of  a  mono- 
rhinic  condition  preceding  the  amphirhinic,  i.e.  with  paired 
nostrils. 

Vascular  System. 

Now,  in  turning  our  attention  to  the  vascular  system,  we 
shall  find  that  Amphioxus  has  no  heart.  In  any  ani- 
mal with  a  comparatively  well-developed  vascular  system, 
the  presence  of  a  heart  might  be  regarded  as  a  sitie  qua 
non.  This,  however,  is  by  no  means  always  the  case  ;  and 
although,  among  the  Invertebrates,  the  extensive  groups 

46 


IXTERXAL   A.VA  TOMY. 


4; 


of  the  Arthropoda  (Insects  and  Crustacea)  and  the 
Mollusca  are  characterised  by  the  possession  of  a  definite 
iiuiscular  heart,  yet  in  the  various  groups  of  worms  there 
aie  many  which  possess  a  very  elaborate  vascuhir  system, 
while  not  one  of  them  possesses  a  heart.  In  fact,  in  the 
last-mentioned  forms,  the  place  of  a  heart  is  taken,  func- 
tionally, by  contractile  Mood-vcsscls.  And  this  is  the  case 
with  Amphioxus.  Among  the  Vertebrates,  including 
the  Ascitlians,  it  forms  the  unique  instance  in  which  such 
an  acardiac  condition  of  the  vascular  system  is  met  with. 

Lying  below  the  j-jharynx  in  the  endostylar  ccelom,  there 
is  a  blood-vessel  known  as  the  branchial  artery,  which  con- 
tracts more  or  less  rhythmically,  and  corresponds  in  its 
position  and  relations  to  he  heart  and  truncus  arteriosus 
of  the  higher  forms. 


1-5?, 


I' -Aft-    ■ 


i:\ 


|em,  we 
ly  ani- 
jystem. 
bic  qua 
le  ;  and 


f^roups 


Fig.  20. —  niagram   illustrating    the   chief    parts   of    the  vascular    system   of 
.Amphioxus.     (Constructed  after  J.  Ml'i.I.r.R  and  SrHNKlDKR.) 

The  arrows  indicate  the  diri'ction  of  flow  of  the  l)lood.     c/i.  Notocliord.     /lep. 
Hepatic  cii'cum.     af.  Afferent   branchial  vessels  (vascular  bulbils   of  ).  Miiller) 
entering  the  primary  bar'-  from  hr.a,  the  branchial  artery ;  the  efferent  branchial 
vessels  are  seen  emerging  from  the  tops  of  both  primary  and  secondary  bars  and 
'       niiining  into  d.ii.  the  dorsal  aorta.     From  the  dorsal  aorta,  the  blood  enters  the 
cipiliaries  over  the  wall  of  the  intestine  (indicated  by  the  dark  reticular  shading), 
;       and  finally  reaches  s.i.v,  the  sub-intestinal  vein.     The  latter  carries  the  Llood  to  the 
;       base  of  the  hepatic  coecuni,  over  which  it  passes  into  another  system  of  caiiiilaries 
\       (nut  indicated),  and  is  then  collected  into  h.v,  the  hepatic  vein,  which  passes  back- 
wards and  curves  round  into  the  branchial  artery. 

I        I'roni  this  branchial  artery,  lateral  branches  running  up 

%. 

%.     into  the  primary  bars  of  the  pharynx  are  given  off  on  both 
I    sides  alternately.     (Cf.  Fig.  20.)     There  appears  to  be  no 


m,i 


"^fT 


48 


ANATOMY  OF  AMPIIIOXUS. 


direct  communication  between  the  vessels  of  the  tongue- 
bars  and  the  branchial  artery. 

At  the  base  of  the  primary  bars  the  lateral  offshoots  of 
the  branchial  artery  are  found  to  be  enlarged  to  form 
vascular  bulbils,  which  are  also  contractile.  F'urthermore, 
at  this  point  they  divide  into  three  branches  of  smaller 
calibre,  v.fhich  constitute  the  vessels  of  the  primary  bar. 


Fig.  21.  —  Diagram  of  a  section  through  the  pharynx  involving  a  primary  bar 
(to  the  left),  and  a  tongue-bar  (to  the  right),  to  illustrate  the  circulation  in  the 
branchial  bars.     (After  Si'KNGKI..) 

br.ix.  Uranchial  artery,  c.  Cn?loni ;  outside  of  which  is  the  atrial  epithelium. 
c.v.  Cd'lomic  vessel  of  j^riniary  bar.  e.  Endostyle.  c.c.  Endostylar  ca'Iom.  e.i . 
lixternal  vessel,  i.v.  Internal  vessel.  La.  Left  aorta,  r.a.  Right  aorta.  /.  Cavity 
of  pharynx,     t.b.  Tongue-bar. 

(Cf.  Fig.  15.)  One  of  these  branches,  as  we  have  seen, 
runs  up  between  the  coelomic  and  atrial  epithelium,  and 
may  be  called  after  Bo\'eri  the  cw/oinic  vessel,  of  the  primary 
bar  (Figs.  15  and  21).  Another  lies  at  the  inner  cdgQ  of 
the  skeletal  rod,  and  is  the  so-called  external  vessel,  while 
a  third  lies  immediately  below  the  inner  pharyngeal  epi- 
thelium of  the  bar,  and  forms  the  internal  vessel. 


Mil' 


'  seen, 

■n,  and 

rimary         1 

xlire  of         1 

f,  while         '{ 

;al  epi- 


INTEKiyAL  ANA  TOMY. 


49 


The  two  last-named  vessels  only  are  represented  in  the 
toiigue-bars,  and  differ  in  their  arrangement  in  the  latter 
ill  so   far   as    the    external  vessel  is   enclosed   within   the 


SKCIC 


tal  rod. 


The  blood  which  circulates  in  the  tongue-bars  flows  into 
tlicni,  not  from  the  branchial  artery,  but  from  the  primary 
bars  through  the  cross-bars  of  the  pbarynx.  The  vessels 
of  each  gill-bar  unite  above  into  ct  single  efferent  vessel, 
which  conducts  the  blood  into  the  dorsal  aorta  of  cither 
side.  So  that  while  efferent  vessels  issue  alike  from  both 
j)i-imary  and  tongue-bars,  the  afferent  vessels,  which  lead 
the  blood  directly  from  the  branchial  artery  into  the  gill- 
bars,  are  confined  to  the  primary  bars  (Fig.  20).  The 
blood,  having  been  oxygenated  during  its  passage  through 
the  gill-bars,  past  which  a  constantly  renewed  stream  of 
water  is  kept  flowing,  enters  the  dorsal  aorta,  and  is  then 
carried  backwards  to  the  region  of  the  intestine.  The 
two  halves  of  the  dorsal  aorta,  which  we  have  already 
noted  on  either  side  of  the  hyperpharyngeal  groove,  be- 
come united  into  a  common  trunk  behind  the  pharynx,  so 
that  in  the  region  of  the  intestine  there  is  a  single  dorsal 
aorta  (cf.  Fig.  28),  from  which  lateral  branches  arc  given 
off  to  the  wall  of  the  intestine.  These  then  break  up  into 
capillaries,  which  anastomose  freely  together,  and  so  form 
a  perfect  vascular  network  rouiid  the  intestine.  Finally, 
the  blood  emerges  from  this  capillary  system  into  a  large 
vein  lying  below  the  digestive  canal,  the  sub-intestinal  vein. 
Here  it  flows  in  a  forward  direction  until  it  reaches  the 
base  of  the  hepatic  caecum.  At  this  point  the  vein  ajjpears 
to  stop  short,  but  in  reality  breaks  up  into  another  system 
of  capillaries  surrounding  the  liver.^  From  these  again 
the  blood  is  collected  into  the  large  multiple  hepatic  vein 
Ivinu'  above  the  coecum.     Here  it  flows  backwards  as  far  as 


it'tiv. 


w> ' 


"Iff" 


$0 


.IXATOMY   OF  A. V PHI  OX  US. 


the  angle  formed  by  the  caecum  with  the  alimentary  canal, 
where  the  vein  bends  sharply  round  into  the  branchial 
artery,  and  so  the  cycle  is  completed  (Fig.  20).  According 
to  Johannes  Mullek,  the  time  required  for  one  complete 
circulation  of  the  blood  in  Amphioxus  is  one  minute,  and 
in  this  time  any  given  droplet  of  blood  will  have  traversed 

the  whole  body.  Con- 
trary ':o  what  takes  place 
in  the  higher  Verte- 
brates, a  single  contrac- 
tion of  the  heart  {i.e. 
branchial  artery)  in 
r.a  Amphioxus  suffices  for 
a  complete  circulatory 
cycle.'-^ 

The  right  and  left 
dorsal  aortas  differ  from 
one  another  in  respect 
to  the  behaviour  of  their 
anterior  cephalic  termi- 
nations.      At  the  front 

Fig.  22. — Transverse  section  through  re- 
gion of  velum  to  show  diffeience  in  behaviour    end  of    the  pharvUX,   ihc 

GERHAN^'r'  "''  "°'''''"    ^^''^'^'^  ^'°'"  ^"'''  '■^Sht    aorta    opens    out 

ch.  Notochord.    La.  Left  aorta,    w/.  Meta-    Jnto   a  widc  VaSCukir  ex- 
pleur.    II.  Sjiinal  cord.    r.a.  Right  aorta,    t.m.  .  v  •    i.  n       i 

Transverse  muscles  ;  the  septum  (raphe)  which  paUSlOU  whlch  flanks  thc 
divides  these  muscles  into  two  halves  is  no  y^Jmyj  qj-,  |;J^g  rio^ht  sidc 
longer  median,  hut   shifted  towards  the   right  "^ 

side  in  consequence  of  the  fact,  discovered  l)y 
VAN  WljllK.that  thc  right  transverse  muscles 
dwindle  out  and  end  in  ihis  region,  while  the 
left  transverse  muscles  are  continued  into  the 
outer  muscle  of  the  oral  hood.     v.  Velum. 


(Figs.  3  and  22,  r.^r.). 
Johannes  Miiller,  who 
first   figured  this  struc- 


ture, took  it  for  the  an- 
teriormost  aortic  arch  connecting  the  branchial  artery 
directly  with  the  dorsal  aorta. 

However,  according  to  the  recent  researches  of  Professor 


/;■, 


tin 

do 

fca 

tioi 

froi 


INTERNAL   .IX.  I  TOMY. 


51 


rofessor 


I  \\\  VAN  VVijME,  it  would  appear  that  this  so-called  aortic 
arch  docs  not  communicate  with  the  hranchial  artery,  but 
ends  blindly  below  in  the  neighbourhood  of  the  right  meta- 
plcur.  Dorsally,  the  aorta  from  which  this  lateral  arch-like 
outgrowth  occurs,  is  continued  forwards  (not  as  a  simple 
vessel,  but  as  a  complex  of  vessels)  as  far  as  a  peculiar 
sense-organ  known  as  the  groove  of  Hcrtsc/uk,  after  its 
discoverer.  This  groove  lies  in  the  roof  of  the  oral  hood 
to  the  right  of  the  notochord,  and  is  derived  from  the 
pvicoral  pit  of  the  larva  (see  below).      (Cf.  Fig.  ^6.) 

In  front  of  the  sense-organ  this  dilated  continuation  of 
the  right  aorta  communicates  beneath  the  notochord  by 
means  of  a  transverse  vascular  commissure  with  the  left 
aorta,  which  retains  its  small  calibre  and  simple  character 
throughout.  From  the  vascular  complex  of  the  right 
aorta  arise  the  vessels  which  supply  the  buccal  cirri. 

Hitherto  we  have  only  spoken  of  those  blood-vessels 
which  are  related  to  some  part  or  other  of  the  alimentary 
canal.  In  point  of  fact  the  parietal  or  somatic  vessels  of 
Amphioxus,  if  present  at  all,  must  have  a  very  subordi- 
nate physiological  significance.  Their  place  is  taken  by 
lyniph-spaccs,  of  which  there  are  a  great  number  in  various 
parts  of  the  body.  Such  are  the  dorsal  and  ventral  fin- 
chambers,  the  spaces  in  the  metapleural  folds,  spaces  at 
the  apices  of  the  myotomes  and  in  connexion  with  the 
dorsal  nerve-roots,  etc.     (Cf.  Fig.  2.)^ 

The  vascular  system  of  Amphioxus  presents  several 
features  of  great  interest  from  a  phylogenetic  or  evolu- 
tionary point  of  view. 

We  have  seen  that  the  heart  is  in  no  way  differentiated 
from  the  branchial  artery  and  is  therefore  a  simple  tubular 
vessel.  This  is  the  primary  condition  of  the  heart  in  the 
embryos  of  all  the  craniate  Vertebrates.     In  the  latter,  as 


^% 


'^^ 


w 


52 


ANATO.VV  OF  A  MP/// OX  US. 


the  embryonic  development  proceeds,  this  simjile  tubular 
heart  widens  out,  acquires  a  series  of  constrictions,  and 
underj^^oes  a  remarkable  flexure  known  as  the  sii^iuoid 
flexure.  Two  stages  in  the  formation  of  the  sii;moid 
flexure  of  the  heart  of  the  chick-embryo  are  shown  in 
Figs.    23    and    24.     At  a   somewluit    earlier    stage   than 


^'■■■.•■•••••.■••V^\  ^/thi 

^.■.'■■.■.■.■:^iitm^:::A^      . 


au 


r-P  -i 


i^ 


VrV- 


•?'?• 


■«'<' 


tffa 


Jv 1 

■.\ 

Figs.  23  and  24. — Anterior  portions  of  chick-embryos  of  the  38fh  and  48th 
iioiir  of  incubation,  seen  from  below,  to  illustrate  formation  of  lieart.  (After 
Duval.) 

ao.  Right  and  left  aortit- .  an.  Auditory  involution,  c.^.  \'entricular  portion  of 
heart,  tg.  Auricular  portion  of  lieart.  e.  Eye.  //.  Heart,  op.  Primary  optic 
vesicle,  p.f.b.  Primary  fore-brain,  p.m.b.  Primaiy  mid-brain,  p.h.b.  Primary 
hind-brain,  t.a.  Truncus  arteriosus,  v.a.  Vitelline  arteries,  v.v.  Vitelline  vei..^. 
y,  ^,  J,  Transitory  gill-slits. 

that  represented  in  Fig.  23  the  heart  was  perfectly 
straight.  In  this  figure  it  is  still  a  simple  dilated  tube, 
but  no  longer  straight.  It  has  become  bent  outwards 
into  a  U-shape.  At  the  stage  of  Fig.  24  well-marked 
constrictions  (the  indications  of  the  later  division  into 
auricle  and  ventricle,  etc.)  have  appeared  in  the  heart,  and 
the   simple    U-shaped    flexure   of   the   latter   has  become 


'l.r,!l' 


IXTKh'XAI.   AXA  TOMY. 


53 


1 


rfectly 
tube, 
twards 
narked 
into 
rt,  and 
kecome 


complicated  by  tiie  occiinvnce  of  a  further  flexure  in  a 
different  direction,  in  consequence  of  wliicii  the  hinder 
limb  of  the  U  has  been  raised,  so  to  speak,  to  nearly  tiie 
same  plane  as  the  anterior  limb.  The  shape  of  the  heart 
at  this  staije  bears  a  characteristic  resemblance  to  the 
(Ircck  letter  si,L;ma.  The  permanent  condition  of  the 
iieart  in  Amphioxus  therefore  corresponds  to  an  early 
.sla,L;e  of  its  development  in  the  hi^dier  Vertebrates. 

Al^^un,  in  the  craniate  embryo  the  dor.sal  aorta  arises  as 
a  pair  of  vessels  on  either  side  of  the  notochord,  which 
later  fuse  toi;ether  into  one  median  tlorsal  vessel.  (Cf. 
l-ii;.  24.)  In  Amphioxus,  throu<;hout  a  ^i;reat  portion  of 
its  extent,  —  namely,  in  the  region  of  the  i)ha-vnx,  —  the  two 
halves  of  the  dorsal  aorta  remain  permanently  sejiarated 
from  one  another  by  the  dorsal  groove  of  the  pharynx. 
(Cf.  I-'igs.  2  and  28.) 

( )ne  of  the  most  striking  peculiarities  of  the  vascular 
system  of  Amphioxus  is  the  presence  of  the  suh-intestiiial 
vein,  in  its  capacity  as  the  main  venous  trunk  of  the  body. 
It  collects  the  blood  from  the  capillaries  of  the  intestinal 
wall,  and  conducts  it  to  the  base  of  the  liver,  where  it  again 
hrcaks  up  into  capillaries.*  It  acts,  therefore,  physiologi- 
cally, as  a  portal  vein,  while  morphologically  it  is  the 
sitb-intcstinal  vein.  Curiously  enough,  it  is  much  larger  in 
its  posterior  than  in  its  anterior  moiety,  and  in  transverse 
sections  through  the  hinder  region  of  the  intestine  there 
apjjcar  to  be  several  separate  vessels  lying  side  by  side, 
sometimes  as  many  as  six.  These,  however,  if  traced 
hackwards  or  forwards,  are  found  to  anastomose  with  one 

*  In  the  larva  of  Amiihioxus  the  sul)-intestinal  vein  and  branchial  artery 
liirni  (ine  continuuus  I)li)otl-vessel.  Later,  when  the  hepatic  C(L'Cuin  (liver) 
i;ru\vs  out  from  the  ventral  wall  of  the  alimentary  canal,  an  interruption  occurs 
in  the  continuity  of  the  vessel,  through  the  insertion  of  a  capillary  portal  system 
ill  its  course. 


54 


ANATOMY  OF  AMl'HIOXUS. 


-e 


Fig.  25.  — View  of  portion 
of  sub-intestinal  vein  of  Amphi- 
oxus,  to  show  iis  fenestrated 
character  in  the  posterior  re- 
gion.    (After  SCHNEIDKR.) 

a.  Anterior.    /.  Posterior. 


another,  as  shown  in  Fig.  25,  and 
so  there  is  produced  a  fenestrated 
structure  in  the  vein.  The  hepatic 
vein  has  a  similar  fenestrated  char- 
acter, and  this  was  what  was  meant 
by  speaking  of  it  above  as  being 
"multiple." 

The  sub-intestinal  vein  reappears 
in  the  embryos  of  all  the  higher 
fishes  and  Amphibia,  where  it  breaks 
up  into  capillaries  in  the  liver.  In 
these  forms,  however,  it  does  not 
persist  long  as  the  main  venous 
trunk,  but  becomes  replaced  almost 
entirely  by  the  development  of  two 
large  veins,  which  arise  on  either 
side  of  the  dorsal  aorta.  These  are 
the  so-called  cardinal  I'eins.  The 
sub-intestinal  vein  mostly  disappears 
after  the  formati(Mi  of  the  cardinal 
vei.is,  but  persists  as  a  second-class 
vfssel  in  the  lampreys  and  in  some 
sharks,  lying,  in  the  latter,  in  the 
spiral  valve  of  the  intestine.*  More- 
over, its  posterior  portion,  which 
lies  in  the  tail,  persists  as  the  cmtdal 
vein. 

*  The  sal5-intestinal  vein  is  also  persistent  in 
the  following  Uroilele  Amphibia  —  Salaiiuxn- 
dra,  7'ritou,  and  Pleiirodi'les.  (See  F.  llocil- 
STKTTKK.  Hcitragc  zur  Vt'txieichenden  Aiiatoinie 
uuif  Entii'tckijoigsgeschichte  des  Vetieiisystems 
der  Amphibicn  tind  Fisc/ie.  Morph.  Jahrh. 
XIII.     1888.     pp.  1 19-172.) 


INTERNAL   ANATOMY. 


55 


I 


The  same  vessel,  therefore,  which  constitutes  the  main 
venous  trunk  of  the  adult  Amphioxus  performs  the  same 
function  in  the  embryos  of  the  higher  fishes.  We  can  thus 
deduce  a  good  deal  of  evidence  from  a  consideration  of  the 
vascular  system  alone,  pointing  to  the  primitive  and  ances- 
tral character  of  Amphioxus. 

If  we  compare  broadly  the  vascular  system  of  Amphioxus 
with  that  of  a  segmented  worm  like  the  common  earth- 
worm, we  are  at  once  confronted  with  certain  obvious 
superficial  resemblances.  Here,  as  in  Amphioxus,  the 
vascular  system  comprises  two  main  longitudinal  trunks, 
one  lying  above  the  intestine  and  the  other  below  it,  and 
furthermore,  they  are  connected  together  at  intervals  by 
circular  vessels  which  form  complete  rings  round  the 
alimentary  canal  in  the  same  way  as  do  the  vessels  which 
pass  through  the  pharyngeal  bars  of  Amphioxus. 

It  is  only  when  we  come  to  enquire  into  the  direction 
of  flow  of  the  blood  in  the  two  cases  that  we  meet  with  a 
striking  contrast  between  them.  Whereas  in  Amphioxus 
the  blood  flows  in  the  dorsal  aorta  from  before  backwards 
(;X'e  Fig.  20),  and  in  the  sub-intestinal  vein  together  with 
the  branchial  artery,  from  behind  forwards,  in  the  worm,  on 
the  contrary,  these  directions  are  reversed,  and  the  blood 
flows  from  behind  forwards  in  the  dorsal  vessel,  and  from 
before  backwards  in  the  ventral  vessel. 


The  Excretory  System. 

The  excretory  function  is  so  intimately  bound  up  with 
the  circulation  that  a  description  of  the  organs  which 
serve  this  function  follows  naturally  after  the  consideration 
of  the  vascular  system.  The  apparent  absence  of  definite 
excretory  organs  in  Amphioxus  was  for  a  long  time  one  of 
the  greatest  difficulties  in  the  way  of  a  correct  appreciation 


WW 


ii^ 


56 


ANAIVMY  OF  AM  PHI  OX  US. 


ac 


of  the  peculiarities  of  its  organisation.     Thanks,  however, 

to   recent  researches,   it  is  now  known  to  possess   such 

organs  in  luxuriant  abundance. 

From  first  to  last  several  entirely  different  structures 

have    been    credited    with    a   renal   function.     Johannes 

MuLLER  first  discovered 
certain  glandular  epithe- 
lial tracts  in  the  floor  of 
the  atrial  chamber  in  its 
hinder  portion.  These 
cellular  thickenings  are 
distinguished  by  their 
high  cylindrical  cells  from 
the  flattened  atrial  epi- 
thelium which  surrounds 
them.  (Cf.  Figs.  1 1  and 
26.)  Johannes  Miiller  sug- 
gested that  these  groups 
of  cells  might  be  renal 
organs.  His  observation, 
however,  failed  to  find 
general  acceptance  among 
morphologists  for  about 
thirty-tivc    years,    when, 

Fig.  26.  —  Transverse  section  throush  ^"  '^^7^,  W.  Roi.I'H  and 
post-pharyngeal  region  of  youns  incliviJual.  p.^y^  LaNGERIIANS,  WOrk- 
ti)  show  Rroii]-)s  of  renal  cells  in  floor  of 
atrium.  (.Xfter  L.WKK.STER  and  WlM.F.Y.) 
ao.  Aorta,  at.  Atrium,  b.c.  Hody-cavity 
(coelom).  ex.  Central  canal  of  nerve-cord 
[n.c).  d.f.c.  Fin-cavity.  i.m.  Intercu'lic  j^,^(|  aCCCptcd  his  intcr- 
menibrane.      I.m.  and   r.w.   Left   and    riglU 

metapleural  folds,     r.p.  One  of  J.   Miiller's    prctatiou  of  the  bodicS  aS 
renal  p.apillaL".    s.i.v.  Sub-intestinal  vein.  ,  ,    . ,  

renal  organs,  at  the  same 
time  adding  a  careful  histological  description  of  them 
(Fig.  27). 


tnr> 


ing    independently,    fully 
confirmed     his     account 


IXTEKNAL  ANATOMY. 


57 


The  individual  groups  of  cells  have  an  elongated  and 
mdic  or  less  ovoid  shape  with  the  long  axis  parallel  to  the 
long  axis  of  the  body.  According  to  Langerhans  their 
surface  is  ciliated.  Two  kinds  of  cells  enter  into  their 
composition  ;  namely,  large  clear  dilated  cells,  which  are 
separated  from  one  another  by  fine  fibre-like  cells  of 
extreme  tenuity  (Fig.  27).  In  the  latter  the  nucleus  of 
each  cell  is  placed  near  the  free  end  of  the  cell,  while  in 
the  former  it  lies  nearer  the 
l)ase  of  the  cell.  Langerhans 
found  highly  refringent  con- 
cretions in  the  dilated  cells 
which  he  took  for  excretory 
products.  That  these  cells 
have  a  capacity  for  excreting 
waste  matters  has  mure  re- 
cently been  shown  experiment- 
ally by  F.  E.  Weiss.  The  atrial 
epithelium  on  the  pharyngeal      Fig.  27.  -  isolated  cells  from  renal 

paiiilla;    the  larg'.:  cells  contaii)  con- 
hars     has     a     similar     character    cretionsimlicated  by  the  black  bodies. 

to  that  forming  these  curious 

renal  papillcc  on  the  floor  of  the  atrium.  The  distribution 
of  these  papillx  in  the  vicinity  of  the  atriopore  is  very 
irregular  and  variable  and  without  any  regard  to  a  sym- 
metrical disposition.  Although  they  are  undoubtedly  to 
he  regarded  as  a  species  of  renal  organ,  yet  they  could 
not  be  compared  to  any  portion  of  the  excretory  system 
of  the  higher  Vertebrates. 

Another  structure,  or  pair  of  structures,  which  has  been 
considered  to  belong  to  the  category  of  renal  organs  must 
next  be  referred  to. 

This  consists  of  two  funnel-shaped  diverticula  of  the 
atrial  cavity  lying  in  the  dorsal  (subchordal)  coelom  in  the 


h, 


ff '»;  i| 


fM-^ 


w^ 


58 


ANATOMY  OF  AMPllIOXUS. 


■\     !.     |. 


region  of  the  twenty-seventh  myotome,  where  the  pharynx 
ends  and  the  intestine  begins.  They  were  discovered  in 
1875  by  Lankester,  who  called  them  the  atrio-ccclouiic  or 
bnnvn  funnels,  on  account  of  the  rich  accumulation  of 
brown  pigment  in  their  walls.  We  have  already  referred 
to  this  brown  pigment  as  occurring  very  generally  in  the 
atrial  epithelium.     The  brown  funnels  have  the  shape  of  an 


Fig.  28.  —  Plastic  diagram  illustrating  the  positions  and  relations  of  the  atrio- 
coeloinic  funnels.  A  rod  is  passed  through  the  peri-enteric  coelom  into  tlie  sub- 
chordal  (suprapharvngeal)  coelom.     (After  LaNKKSTKR.) 

ao.  Dorsal  aort;v.  at.  Atrial  cavity,  b.f.  Atrio-coeloniic  funnels,  go.  Gonads. 
l.d.  Ligamentum  denticulatum  diharyngo-pleural  folds,  Lankester).  /.///.  and 
r.m.  Left  and  right  metapleural  folds.  my.  Muscles.  fli.  Roof  of  ]jharyn\. 
«.  I'oint  of  union  of  the  right  and  left  aorta'  into  the  median  aorta. 

elongated  cone,  the  apex  of  which  is  directed  forward.*^. 
At  the  wide  end  each  funnel  opens  into  the  atrial  cavity, 
while  at  the  narrow  end  it  is  possible,  but  not  certain,  that 
an  opening  exists  into  the  dorsal  coelom  (Fig.  28).  The 
funnels  are  adherent  throughout  their  entire  length  to  the 
roof  of  the  dorsal  coelom.'* 


INTERNAL  ANATOMY. 


59 


).  Gonads. 
l.vi.  and 
pharynx. 


rwards. 

cavity, 
in,  that 
Tlic 

to  the 


In  1889  Wkiss  undertook  the  task  of  determining  ex- 
perimentally whether  Johannes  Muller's  renal papilUc  and 
Lankester's  brozvn  fuiuich  really  served  an  excretory 
function.  The  method  of  research  consisted  in  feeding 
full-grown  individuals  with  various  colouring  matters  held 
in  solution  or  in  suspension  in  sea-water.  For  instance, 
carmine  suspended  in  sea-water  would  be  carried  into  the 
digestive  canal  and  then  absorbed  through  the  intestinal 
epithelium  into  the  capillaries  surrounding  the  intestine. 
It  would  thus  get  into  the  vascular  system,  and  also  by 
some  means  into  some  of  the  lymph  spaces,  and  finally 
would  be  excreted  by  the  cells  of  the  renal  papillae  or  by 
whatever  other  structure,  or  set  of  structures,  might 
possess  the  renal  function.  In  fact,  Weiss  found  that  the 
so-called  renal  papillae  did  actually  excrete  a  quantity  of 
the  carmine  with  which  the  animals  had  been  fed,  and, 
further,  that  a  similar  excretion  of  carmine  occurred  at 
other  points  of  the  atrial  epithelium.  The  atrial  epi- 
thelium, as  a  whole,  probably  has  more  or  less  the  power 
of  excreting  waste  products  which  have  found  their  way 
into  the  vascular  and  lymphatic  systems. 

But  above  all,  Weiss  discovered  a  very  active  excretion 
of  carmine  in  certain  small  tubules  which  he  found  lying 
in  the  dorsal  ccelom  applied  against  the  most  dorsal  por- 
tion of  the  double-layered  membrane  (ligamentum  denti- 
culatum)  which  separates  the  coelom  from  the  atrial  cavity 
( JMg.  29).  There  is  one  of  these  tubules  to  each  primary 
gill-cleft  of  the  pharynx.  At  the  top  of  each  tongue-bar 
Weiss  made  out  an  opening  of  the  tubule  into  the  atrial 
cavity,  but  he  did  not  succeed  in  finding  any  openings  into 
the  dorsal  coelom.  After  the  operation  of  feeding  with 
carmine  was  completed,  at  the  close  of  a  week  or  fortnight, 
and  time  had  been  allowed  for  its  absorption  and  subse- 


K,,.-* 


■■'h 


T^\i..\ 


6o 


ANATOMY  OF  AMPHIOXUS. 


qucnt  excretion,  the  epithelium  lining  the  walls  of  these 
tubules  was  found  to  be  full  of  carmine  granules. 

At  about  the  same  time  at  which  Weiss  was  pursuing 
his  studies  on  Amphioxus  Theudor  liovEKi,  having  been 
led  by  independent  a  priori  considerations,  largely  induced 
by  the  work  of  Rlckekt  on  the  development  of  the  ex- 
cretory system  of  Selachians,  to  suspect  the  occurrence 


^.^.s.ch 


Fig.  29.  —  Portion  of  transverse  section  through  the  pharynx  of  Aniphioxu.-, 
to  show  position  of  excretory  tubule.     (After  W'l'.iss.) 

ao.  Left  .lorta.  at.  Atrial  cavity,  at.c.  Atrial  epithelium.  ■  c.  Cu,'loni.  ch.  Notu- 
chord.  i.in.  Intercoelic  iiieiubrane.  l.d.  Ligamentum  denticulatum.  «//r.  Kxcit- 
tory  tubule,  p.b.  Primary  bar.  pit.e.  Epithelium  of  hyperpharyngeal  groove. 
ph.  f.  Pharyngo-pleural  fold,    s.cli.  Sheath  of  notochord.     t.b.  Tongue-bar. 

of  excretory  tubules  in  Amphioxus  comparable  to  those 
found  in  the  embryos  of  the  higher  Vertebrates,  instituted 
a  search  for  them  and  discovered  them  independently  in 
the  most  brilliant  manner. 

Boveri  carried  his  investigation  to  a  high  pitch  of  per- 
fection, and  has  published  an  account  of  these  tubules, 
which  in  point  of  clearness  and  completeness  leaves  nothing; 


^! 


I 


Fig. 

of  th,. 
uaii  of 


yn.v 
callv, 
!n-  a 


IXTERNAL  AXATOMV. 


6l 


til  be  desired.  The  accompanying  figures,  taken  from 
I'lucri's  finely  illustrated  memoir,  show  the  appearance 
and  topographical  relations  of  the  excre:ory  tubules. 

A  tubule  as  seen  in  the  living  condition  is  shown  in 
Im-.  30.     It    is  a  curved  tube  consisting  mainly  of  two 


l*'^' 


i9| 


Fig.  30. — An  excretory  tubule  of  the  left  side,  with  the  neif^hbourint^  portion 
of  the  pharyngeal  wall,  as  seen  in  the  living  condition.  The  round  bodies  in  the 
'.v.iii  of  the  tubule  represent  earniine  granules.    Highly  magnified.    (After  Ro\KRl.) 


i£>^«#' 


r\-i 


h:-. 


no 


thini 


limbs,  bent  approximately  at  right  angles  to  one  another, 
and  lying  over  against  the  dorso-lateral  wall  of  the  jihar- 
yiix,  (Cf.  Fig.  29.)  The  anterior  limb  is  directed  verti- 
cally, and  the  posterior  longitudinally.  The  former  opens 
l\v  a  relatively  wide  and  forwardly  directed  opening  into 


y^^ 


ili 


62 


A.V.I  JVM y   OF  .I.MI'HIO.XUS. 


the  dorsal  ccelom.  The  posterior  end  of  the  tube  also 
opens  into  the  ccelom,  and  between  these  two  terminal 
openings  there  is  a  variable  number  of  other  ccclouiic 
opctiings,  or  funnels,  as  they  are  called,  situated  on  the 
dorsal  side  of  the  tubule,  and  opposite  to  that  side  which 
carries  the  opening  into  the  atrial  chamber.  The  coelomic 
funnels  are  placed  at  the  ends  of  short  upstanding  projec- 
tions from  the  main  body  of  the  tubule.  On  the  ventral 
side  of  the  tubule,  opposite  in  each  case  to  a  tongue-bar  of 
the  pharynx,  occurs  the  single  opening  into  the  atrial  cav- 
ity. The  epithelium  lining  the  tubule  consists  of  cubical 
ciliated  cells.  There  is  a  thick  bunch  of  cilia  in  connec- 
tion with  the  atrial  opening  of  the  tubule.  The  curious 
thread-like  structures,  carrying  a  round  knob  at  their  dis- 
tal extremities,  which  radiate  out  from  the  coelomic  open- 
ings, are  specially  modified  cells  belonging  to  the  ccelomic 
epithelium,  which  are  probably  concerned  in  promoting 
the  excretory  activity  of  the  tubule,  and  are  called  by 
Boveri,  thread-cells  (Fadenzellen). 

The  vascular  supply  and  exact  location  of  the  nephridial 
tubules  (each  tubule  representing  a  nephridium,  according 
to  Lankester's  nomenclature)  are  shown  in  Fig.  31.  The 
figure  represents  a  piece  of  the  upper  wall  of  the  pharynx, 
cut  out  in  such  a  way  as  to  expose  the  inner  wall  of  the 
dorsal  coelom.  The  cross  is  placed  at  the  cut  edge  of  the 
double-layered  membrane  which  separates  the  dorsal  coelom 
from  the  atrial  cavity.  This  cut  c^^q  can  be  traced  from 
side  to  side  of  the  figure.  The  membrane  is  seen  to  be 
continued  down  each  primary  gill-bar,  in  company  with  the 
extension  of  the  coelom,  which  runs  down  the  primary  bars 
into  the  endostylar  cotlorn  as  described  above.  On  the 
other  hand,  the  membrane  skips  over  the  tongue-bars,  so 
that  the  atrial  cavity  is  prolonged  dorsalwards  into  a  deep 


LYTEKXAL   AAA  TOM  \  \ 


63 


bay,  corresponding  to  each  tongue-bar.  (Ct".  Fig.  29.) 
Thi.s  is  what  produces  the  sinuous,  or  notched,  appearance 
to  the  membrane  in  question,  and  led  Johannes  Miiller  to 
speak  of  it  as  the  ligaincuPittn  dcnticnlatum.  (Cf.  Fig.  28.) 
The  external  or  atrial  opening  of  the  tubule  lies  against 
the  tongue-bar  at  the  head  of  this  bay-like  extension  of  the 
atrial  cavity  (Fig.  31  on  the  right). 

The  vascular  supply  of  the  tubules  is  effected  in  each 
case  by  the  co-operation  of  two  blood-vessels  ;  namely,  the 


Fig.  31.  —  Plastic  figure  illustrating  the  blood-supply  (glomeruli)  of  the  excre- 
torv  tubules.  On  the  right,  the  drawing  is  taken  at  a  deeper  level,  to  show  the 
atri.il  opening  of  the  tubule  over  against  a  tongue-bar.     (After  Hovkki.) 

^.  C"ut  edge  of  ligamentum  denticulatum.  c.v.  Ciiiloniic  vessel  of  primary  bar. 
e.v.  K\ternal  vessel,     i.v.  Internal  vessel,     d.a.  Left  dorsal  aorta. 


%: 


civlomic  vessel  oi  the  primary  bar  (cf.  Figs.  15  and  21)  and 
the  external  vessel  of  the  secondary,  or  tongue-bar.  As 
soon  as  the  coelomic  vessel  of  a  primary  bar  arrives  at  the 
level  of  a  tubul^^,  it  gives  off  a  number  of  branches,  which 
not  only  anastomose  among  themselves,  but  become  united 
with  a  similar  series  of  anastomosing  vessels  which  origi- 
nate from  the  external  vessel  of  the  next-following  tongue- 


M^. 


FF"^ 


64 


AXATOMY   OF  AMI>//IOXUS. 


bar.  In  this  way,  a  complicated  plexus  of  blood-vessels  is 
formed  around  and  about  the  tubule.  This  vascular  ple.xus 
is  known  as  :x  g/oincniiiis. 

The  blood  charged  with  whatever  waste  matters  it  may 
have  gathered  up  in  its  course  through  the  body  arrives 
eventually  at  the  glomeruli,  where  it  is  considerably 
delayed  on  account  of  the  vascular  plexus  through  which 
it  has  to  pass  before  reaching  the  dorsal  aorta.  During 
this  delay,  it  is  exposed  to  the  glandular  excretory  action 
of  the  tubules,  by  which  the  waste  products  are  extracted 
from  the  blood  by  osmotic  action.  F'rom  the  glomerulus 
the  blood  is  conducted  by  two  efferent  vessels,  corre- 
sponding respectively  to  the  primary  and  tongue-bars, 
into  the  dorsal  aorta.  The  communication  between  two 
neighbouring  glomeruli,  as  shown  in  Fig.  31,  is,  according 
to  Boveri,  the  exception  and  not  the  rule. 

The  distribution  of  these  remarkable  excretory  tubules 
or  nephridia  is  coextensive  with  that  of  the  pharyngeal 
gill-clefts.  They  extend  from  the  anterior  to  the  posterior 
extremity  of  the  pharynx,  but  not  beyond  this.  They 
never  have  more  than  one  opening  into  the  atrial  cavity, 
but  those  occurring  in  the  mid-region  of  the  pharynx  have 
several,  sometimes  as  many  as  nine,  openings  into  the  dor- 
sal coelom.  The  number  of  coelomic  openings  decreases 
anteriorly  and  posteriorly,  until,  at  the  two  extremities 
of  the  pharynx,  there  is  only  a  single  coelomic  opening 
to  the  tubules. 

In  a  full-grown  individual,  Boveri  has  counted  ninety- 
one  tubules  on  one  side  of  the  pharynx,  the  total  number 
therefore  being  double  this. 

The  serial  distribution  of  the  excretory  tuuules,  one 
after  the  other,  is  known  broadly  as  a  Dictamcric  arrange- 
ment.    But  since  they  correspond  in   number  and  situa- 


i 


ii 


r  ''<^" 


IXTEKA'AL   AXA  TO.VV. 


65 


tion    to   the    primary   gill-clefts,    which    are    much 


miMc 


^al 


:ics 


ing 


3cr 


ige- 


tua- 


th: 


th( 


)f  the  bodi 


.Tous  man  the  myotomes 
in  which  they  occur,  their  arrangement  is  more  strict!}' 
(Ufineil  as  brafichiouuvic.  In  the  larva,  however,  the  pri- 
mary gill-slits  correspond  numerically  with  the  myotomes 
wr  muscle-segments  of  the  pharyngeal  region,  only  scc- 
oiularily  becoming  more  numerous.  The  branchiomeric 
arrangement  of  the  excretory  tubules  of  Amphioxus  need 
not,  therefore,  prejudice  their  claim  to  be  regarded  as 
Siij-maital  structures. 

If,  now,  we  attempt  to  compare  the  nephridial  system 
(if  Anijjhioxus  with  the  kidney  of  the  higher  tyjies,  we 
shall  find  that  here  also,  as  in  so  many  other  instances, 
the  permanent  state  of  things  in  the  former  becomes  a 
characteristic  feature  of  the  embryo  in  the  latter. 

As  is  well  known,  the  kidney  of  the  higher  Vertebrates 
C(inij)riscs  a  mass  of  convoluted  tubules,  the  urinifiyoiis 
tnbiilts,  imbedded  in  a  matrix  of  fibrous  connective  tissue, 
and  enclosed  within  a  common  sheath,  and  so  producing 
collectively  a  compact  organ  which  we  call  the  kidney. 

If,  neglecting  the  highly  elaborate  structure  presented 
by  the  kidney  of  Birds  and  Mammals,  we  take,  as  a  typi- 
cal example  of  a  primitive  Vertebrate  renal  organ,  that  of 
a  tailed  Amphibian,  we  find  after  a  superficial  examina- 
tion the  following  characteristic  features.  In  the  newt, 
tor  instance,  the  surface  of  the  elongated  kidney  is  studded 
wilh  numerous  small  apertures.  These  are  surrounded  i)y 
vibratile  cilia,  and  lead  directly  from  the  body-cavity  into 
the  convoluted  renal  tubules.  They  arc,  therefore,  the 
co'lomic  openings  or  funnels  of  the  latter,  and  are  known 
as  }uplirositomcs.  Close  to  the  nephrostome  a  short  diver- 
ticulum of  the  tubule  leads  to  a  capsule  which  encloses  a 
glomerulus.     After  a  winding  course  in  the  substance  of 


-^-7^^ 


66 


ANAJOMV   OF  AMnilOXiS. 


the  kidney,  the  tubules  emerge  from  the  latter  as  a  series 
of  efferent  ducts  placed  one  behind  the  other,  and  these 
again  open  into  a  common  longitudinal  duct  on  each  side 
of  the  body,  known  as  the  iircti'i;  which  leads  the  products 
of  excretion  backwards  to  the  cloaca. 

The  permanently  functional  kidney  of  Fishes  and  Ani- 
j)hibia  is  known  as  the  mcso)U'phivs.  In  Reptiles,  Biids, 
and  Mammals,  this  is  only  functional  during  the  embryonic 
period,  and  later  is  replaced  in  a  way  not  yet  fully  eluci- 
tlated  by  the  permanent  kidney  of  these  forms  which  is 
known  as  the  nictancphros. 

The  ureter,  or  duct,  of  the  mesonephros,  is  spoken  of  as 
the  tncsoncphric  duct,  while  the  renal  tubules  constitute, 
collectively,  the  glandular  portion  of  the  kidney. 

The  permanent  kidney  of  the  craniate  Vertebrates  is  ab- 
solutely unique  among  all  the  other  glands  of  the  body,  in 
the  fact  that  the  glandular  portion  of  the  organ  arises 
independently  of  the  duct,  and  only  communicates  saoii- 
darily  with  it.  Moreover,  the  duct  develops  in  point  of 
time  before  the  gland.  This  is  a  very  extraordinary  fact, 
and  taken  alone  would  be  quite  inexplicable.  It  has  been 
found,  however,  that  the  mesonephric  duct  has  primarv 
relations  with  a  totally  distinct  set  of  excretory  tubules, 
which  differ  from  those  mentioned  above,  both  in  their 
position  in  the  body  and  in  their  mode  of  development. 
These  primitive  tubules,  which  mark  the  first  appearance 
of  a  renal  organ  in  the  Vertebrate  embryo,  constitute  the 
pronephros. 

The  degree  of  development  attained  by  the  pronephros, 
or  primitive  kidney,  in  the  life-history  of  the  various  types 
of  Vertebrates,  is  very  different  in  the  different  classes. 

Frequently,  as  with  the  Selachians  (sharks),  Birds,  most 
Reptiles,  and  with  the  Mammals,   the  pronephros  is  an 


ri 


INTEKXAL   AX  A  TOAfY. 


(>7 


I 


lost 
an 


entirely  rudimentary  structure,  which  puts  in  a  tleeting 
ai)i)earance  ilurinj;  the  embryonic  development,  but  never 
functions  as  a  kidney. 

In  other  cases,  as  with  the  Teleostomes,  or  bo.iy  fishes, 
Amphibians,  Crocodiles,  and  Turtles,  the  pronephric  sys- 
tem attains  a  hii^her  grade  of  development,  and  actually 
functions  for  a  time  as  the  sole  kidney  of  the  animal.  In 
sDUic  of  the  bony  .".shes  (e.g.  Zoarces  and  Mcrlncius),  it 
functions  as  the  kidney  for  an  extraordinarily  long  time, 
apparently  throughout  the  period  of  adolescence.  In  one 
curious  instance  of  a  fish,  Ficmsfer,  which  has  acquired  a 
semi-parasitic  habit,  it  appears  that  the  development  has 
been  arrested  to  such  an  extent  that  the  pronephros 
lunctions  as  the  principal  organ  of  excretion  throughout 
life,  the  mesonephros  remaining  rudimentary  (Kmkkv). 

The  most  extensive  pronephric  system  which  has  as  yet 
been  described  for  any  craniate  Vertebrate,  is  that  repre- 
sented diagrammatically  in  Fig.  32.  This  is  the  larval 
excretory  system  of  a  remarkable  worm-like  legless  Am- 
phibian, Ichthyophis  glutinosus,  belonging  to  a  very  primi- 
tive subdivision  of  the  Amphibia  known  as  the  Caxiliani, 
which  occur  in  the  hot  regions  of  South  America,  Africa, 
Seychelles,  East  Indies,  and  Ceylon. 

We  owe  our  Knowledge  of  this  elaborate  pronephric 
system  to  Richard  Semon  of  Jena. 

It  consists  of  some  twelve  pairs  of  irregularly  contorted 
tul)ules  placed  dorsal  to  the  general  body-cavity  in  a  posi- 
tion which  is  described  as  retro-peritoneal,  and  arranged  scg- 
mentally,  one  behind  the  other,  on  either  side  of  the  dorsal 
aorta.  Broadly  speaking,  the  canals  run  outwards  in  a 
transverse  direction.  Near  their  inner  extremities  they 
usually  divide  into  two  short  branches,  which  terminate 

ich   in   a   funnel-shaped   opening   into   the   body-cavity. 


J 


■^^ 


68 


A.y.r/D.u]'  OF  AMPinoxrs. 


a 


■ill 


Hv-.-iii! 


i 

Fig.  32.  —  Pronephric  system  of  einbrvc^  ot  Icluliyoi)liis,  ircunstniclfd  from  sec- 
tions, and  rcpn-sented  as  havint;  been  siinad  out  in  one  iiLme.     (Altur  Simmon,  i 

a.  Dorsal  aorta,  c.  I'ortions  of  the  i;(_i.'lom  Into  wliicli  the  neplirostomcs  of  the 
pronephric  tul)ules  open.  The  inner  portion  of  ca'loni  (next  to  a(>rta)  is  shut  off 
fron*  tlie  rest  of  the  coehim,  and  becomes  associated  witli  the  vascular  outgrowths 
from  the  (h^rsal  aorta  (wliich  produce  the  (glomeruli)  to  form  the  ^hdp'ghian  cap- 
sules of  the  ])ronephvos.  The  Malpighian  tractus  is  continued  backwards  a- 
a  m  'tamotiihosed  and  ."udimeiitary  cord  of  ci'lis,  nearly  to  the  cloaca,  and  con- 
stitutes '.he  so-called  Nebeniiiere  or  Interrenal  body.  This  bacKwaid  extcnsii  i. 
of  tie  .\Iaipii;hian  liodv  of  the  jironephros  p.  bably  indicates  the  former  existenn 
of  1  much  more  extensive  pronephric  systen..  />.  Convoluted  pronejihric  tubui("- 
Ivi'it;  above  the  iieritonmim  'sluulet!  Ii<;ht),  each  provided  with  two  nephrostomes, 
inner  and  outer,  and  openini;  |H'rip  lerally  into  d,  the  longitudinal  ])roni  phric  dun 
(W'olttian  duct),  which  becomes  the  mesonejj  iric  duct  after  the  (kgencrr.tioii  it 


ihe  i)ronephric  tubules  and  the  form.ition  of  the  mesone|)hric  tubules  liave  taJiiu 
place,      w.  Rudiments  of  the  mesonepliric  tubules. 

\.H. —  The  pronephrii    tubules   :'.ie   here  clKir.icterised   by  the    ]iu '-sessio'i    ot 
coL'cal  outgrowth:. 


/xj'i-:k\al  axa  tomy. 


69 


These  are  the  coelomic  openings,  or  nephrostomes,  of  the 
tubules.  At  their  outer  ends  most  of  them  open  directly 
into  a  longitudinal  duct,  \.\i^  pronepJiric  duct,  which  extends 
backwards  to  the  cloaca. 
The  most  anterior  tubules, 
however,  tend  to  fuse  10- 
nether  at  their  outer  ex- 
tremities, before  reaching 
the  common  duct.  Corre- 
sponding to  each  tubule 
tliere  is  a  short  artery 
urowing  out  from  the  dor- 
sal  aorta,  anc'  abutting  with 
its  blind  end  against  the 
portion  of  the  body-cavaty 
into  which  ti  e  innermost 
iKpiirostomes  open. 

Later  on  these  coecal 
outgrowths  from  the  dorsal 
aorta  develop  a  vascular 
network  at  their  free  ends, 
and  so  produce  a  series  of 

glO)Hi  1  Ull.  pjg    33._SchcniatiL'  tr.\ns verso  section 

If,   now,  we   inquire    into    thnMi^h  i  selachian  eml.iy.)  in  the  region 

of  the  pronephros,     (.\fter  van  WlJIlK.) 
the    mode    of    development  The  dotted  line  drawn  across  the  section 

/•  1  1      •  indicates  the  plane  of  division  between  the 

of    such    a    pronephnc    SyS-    „|,|,„^    ,,„nLwd    and    .he    lower    nr.seg- 

tom    as    the    one   above    de-    m-nted  pniti,)ns  of  tlie  primitive  hodycavity 

(procu'loni ).     my.   Mvotonie  or  nivoniere. 

scribed,     we     find     that     its    ms.  Mesonu-re  ,ir  nephrotome.    /.'  I'n.ne- 

„        .    .     \      ^  •  iihrie  o\i'<irowth.     sp.  Unsetrniented    hndv- 

component  tubules  arise  as  \^^.^^,  .,%,,,„,i,„U.i.    '■.  Sclerotome. 

a    series    of    knob-like    Seg-    "•  N>-rve-tulje.     <■/,.    Notochord.     a^.   Dor- 
sal aorta,     al.  Digestive  tube. 

mental     outgrowths     from 

tlic  outer  or  somatic  layer  of  the  peritoneum  at  the  base 

"f    the    segmented   portion   of   the  primitive  body-cavity 


)•^    ot 


70 


AA'ATOMV  OF  AMPHIOXL'S. 


These  outgrowths  arc  at  first  solid  cell-proliferations  of 
the  peritoneal  epithelium,  in  the  midst  of  which  a  lumen 
is  subsequently  formed  between  the  cells.  As  soon  as 
this  occurs,  the  peritoneal  thickenin<;s  rc[)resent  hollow 
diverticula  of  the  coelom,  each  communicating  with  the 
latter  by  a  single  nephrostome  (F'ig.  33). 

The  incipient  tubules  then  grow  outwards  until  J.iey 
reach  the  ectoderm  with  which,  in  the  Selachians,  they 
becone  fused.  This  has  been  taken  by  Riickert  to  indi- 
cate that  the  tubules  originally  discharged  the  product: 
of  excretion  directly  to  the  exterior  by  a  series  of  indepen- 
dent apertures  at  the  points  of  fusion.  (Cf.  Fig.  34  yi.)'' 
The  pronephric  tubules  next  commence  gradually  to  relin- 
quish their  coalescence  with  the  ectoderm  from  before 
backwards,  retaining,  iiowever,  for  the  present  the  connec- 
tion behind  (Fig.  34  B). 

Meanwhile  the  distal  ends  of  the  successive  tubules 
undergo  confluence  (Fig.  34  B),  and  in  this  way  the  begin- 
ning of  a  longitudinal  duct  is  produced.  This  duct  riow 
gradually  splits  itself  off  from  the  ectoderm,  so  that  the 
posterior  connection  with  the  'atter  is  carried  farther  and 
farther  back  until  it  reaches  the  region  of  the  cloaca,  wh(;n 
it  leaves  the  ectoderm  and  acquires  an  opening  into  the 
cloaca  (Fig.  34  C).  Meanwhile,  however,  in  the  Sela- 
chians, the  pronephric  tubules  begin  to  undergo  a  retro- 
gressive development  and  atroj)hy,  as  a  consequence  of 
which  the  pronephros  as  a  gland  becomes  aborted. 

In  the  same  wa}',  but  at  a  much  later  stage,  the  remark- 
able ]-)roncphric  system  of  Ichthyophis  becomes  entirely 
aborted.  But  the  duct  remains,  and  a  r  ^w  set  of  tubules 
appear  at  the  bases  of  the  somites,  which  secondarily  open 
into  it  (Fig.  34  C). 

These   new  tubules  are  the  uicsoncphric  tubules,  and, 


y 


/A'  TERNAL  AX  A  TOMV. 


71 


although  they  occur  mostly  behind  the  region  of  the  pro- 
nephros, yet  rudiments  of  them  appear  in  the  same  seg- 
ments occupied  by  the  latter.  Unlike  the  pronephric 
tubules,  they  arise,  not  as  evaginations  from  the  base  of 
the  somites,  but  in  such  a  way  that  an  adjacent  portion 
ut  the  somite,  lying  dorsal  to  the  pronephric  tract,  loses 


en 

k1. 


N 


I 


Ul  7\ 


yin 


\ 


C 
-met 

—fnn 


— «e 


Fig-  34- — Tlirec  cli.ipmnis  illustiating  tlie  hypmhetical  phylogenetic  develop- 
ment (it  tlie  L'.xcretovy  organs  in  Selacliians.     (.Mtor  Hi CKKR 1. ) 

s.  Somites,  pn.  Pronephric  tubules  fused  with  ec,  the  ectoderm  in  A  ;  collected 
into  a  common  duct  xu.d,  the  Wolttian  or  pn^nephric  (Uict  in  // ;  and  ti-uilly 
aborted  in  ( ',  with  ttic  exception  of  one,  wliicii  persists  as  the  ostiiuu  abdominale. 
mil.  Mesonejihric  tul)ules.  luJ.  Pronephric  duct  in  />';  mesonephric  duct  in  C. 
cl.  Cloaca.    />.  Posterior  region. 


its  primary  connection  with  the  rest  of  the  somite,  which 
consists  of  the  myotome  projier,  and  becomes  bodily  con- 
\erted  into  a  mesonephric  tulnde  whose  blind  end  curves 
round  the  pronephric  duct  and  eventually  opens  into  it  ; 
while  its  point   of  communication  with  the  unsegmented 


n 


ANATOMY  OF  AMPHIOXCS. 


\i 


I 


body-cavity  persists  as  the  ncplirostomc.  (Cf.  Figs.  33 
and  35  B) 

The  proncphric  duct,  therefore,  becomes  secondarily 
employed  in  the  surface  of  the  mesonephros.  So  that, 
while  the  mesonephros  and  its  future  duct  form  two  dis- 
tinct morphological  structures,  the  pronephros  and  the 
same  duct  form  one  inseparable  whole. 

I'^-om  the  above  considerations  we  may  conclude  that 
the  pronephros  rep'"esents  the  primitive  and  ancestral 
excretory  organ  of  the  craniate  Vertebrates.  Just  as  the 
notochord  has  been  largely  replaced  first  by  rnri-ilage  and 
then  by  bone,  so  the  pronei)hros  has  been  replaced  first 
by  the  mesonephros  and  then  by  the  metanephros. 

Returning  now  to  Amphioxus,  we  have  to  note  in  the 
first  place  the  absence  of  a  common  matrix  surrounding 
the  excretory  tubules,  and,  secondly,  the  absence  of  a  com- 
mon duct.  Since  in  the  higher  Vertebrates  the  interstitial 
growth  of  connective  tissue  among  the  tubuks,  binding 
them  together  into  a  compact  organ,  is  a  secondary  phe- 
nomenon, the  absence  of  such  a  matrix  in  Amphioxus 
need  not  detain  us. 

Judging  from  the  analogy  of  the  other  systems  of  or- 
gans in  Amphioxus,  it  will  be  at  once  concluded  that  the 
excretory  tubules  of  the  la^^^ter  represent  the  pronephric 
system  of  the  embryos  of  the  craniate  Vertebrates.  And 
this,  in  fact,  is  Boveri's  contention. 

As  we  have  seen,  the  excretory  tubules  of  Amphioxus 
open  separately  into  the  atrial  cavity.  While  they  do  not, 
therefore,  open  directly  to  the  exterior  at  the  ectodermic 
surface  of  the  body,  they  do  actually  open  at  an  ecto- 
dermic surface,  since  the  atrial  cavity  is  a  space  enclosed 
from  the  outside,  and  so  is  lined  by  ectoderm.  The  pri- 
mary fusion  of  the  pronephric  tubules  with  the  ectoderm, 


ntL 


CO- 


af- 


9' 


ai:c\ 


A 


IXTKKXA/.    AXATOMV. 


n 


which  has  been  observed  in  some  craniate  Vertebrates  as 
described  above,  is  therefore  probably  of  the  same  nature 
as  the  ectodermic  openings  of  the  tubules  in  Amphioxus. 


,f% «f/ 


-  <sc 


-  ch 


■  a-ci 


CO 


J*  J^     e-c 


Fig.  35. — . /.  Sclioiuatic  transverse  section  throiii;h  i)liarvn<;eal  region  of  Ani- 
piiioxu.-i.     On  tlie  left  is  a  lirancliial  bar,  cut  lengthwise,  and  on  tiie  right  a  gill-slit. 

/.'.   Si-lieinatic  transverse  section  through  Selachian  enil)ryo.      (After  Hi )\l'.KI.) 

(j/.t.  Atrial  chamber,  p.n.d.  i'ronephric  duct.  c.o.  Nephrustonie  of  proncjihric 
liibiilc.  k.f.  Cross-section  of  excretory  tubule  in  Ainjiliioxtis.  <t.f.  Opening  of 
excretory  tuliule  into  atrium  in  Ani[ihioxus.  ,^''.(\  (lonadic  cavity  (])erigonadial 
cii'ioni)  in  ,  / ;'  compared  by  Hovcri  with  the  mesoncphric  tubule,  »;rs,f.  in  />'. 
gl.  Glomerulus,  cu-.  C'uelom.  e.c.  I'Lndostylar  ccelom.  s.i.v.  Hranchial  artery  in 
./;  sub-intestinal  vein  in  H, 

Other  letters  as  in  jirevious  figures. 

X.H. —  In  Ji  the  future  o])eiiing  of  the  mescmejihric  tulnile  into  the  pronejiliric 
duct  is  indicated  bj  dotted  lines  on  the  right.  The  vessel  connecting  the  sub- 
intestinal  vein  with  the  aorta  is  placed  on  the  left  of  tl:e  alimentary  canal  for  com- 
parison with  Fig.  A.  It  is  really  only  present  on  the  riglit  side,  althougli  a  riiflimcnt 
occurs  on  the  left.     (See  Note  6.) 

The  glomeruli  of  the  tubules  in  Amphioxus  are  sapiilicd 
by  blood-vessels  which  connect  the  dorsal  aorta  with  the 
branchial  artery.  It  should  be  remembered  that  the  bran- 
chial artery  represents  the  anterior  portion    of   the    sub- 


74 


.■I.V.IVOJ/V   OF  AMPHIOXUS. 


intestinal  vein,  and  in  the  young  larva  the  two  vessels  are 
continuous.  The  direct  continuity  is  subsequently  inter- 
rupted by  the  development  of  the  hepatic  ccecum,  and  the 
consequent  insertion  of  a  capillary  portal  system  into  the 
circulation.  In  the  Selachian  embryo,  a  series  of  similar 
vessels,  six  in  number,  connecting  the  dorsal  aorta  with 
the  sub-intestinal  vein,  have  been  shown  to  be  in  close  coi"- 
respondence  with  the  i)ronephric  tubules,  and  to  form  at 
the  level  of  the  tubules  a  series  of  rudimentary  glomer- 
uli (1^'igs.  35  A  and  B)}' 

Such  resemblances  as  the  above  are  demonstrative,  and 
are  sufficient  to  prove  that  the  excretory  tubules  of  Am- 
l^hioxus  belong  to  the  pronephric  system,  and  that  in  this 
respect,  also,  the  adult  Amphioxus  presents  features  which 
are  characteristic  of  the  embryos,  or  larvae,  of  the  higher 
forms. 

Although  convinced  as  to  the  essential  identity  of  the 
excretory  tubules  of  Amphioxus  with  the  pronephros  of 
the  craniate  Vertebrates,  it  must  be  remembered  that 
there  is  one  apparently  great  difference  between  them. 
Whereas  in  Amphioxus  the  pronephros  (applying  this 
term  to  the  tubules  considered  collectively)  occurs  in  the 
region  of  the  perforated  pharynx,  in  all  the  higher  Verte- 
brates it  occurs  behind  the  pharynx,  and  is  quite  absent 
from  the  region  of  the  gill-slits.  This  difference,  however, 
which  might  at  first  sight  appear  serious,  is,  in  reality, 
most  instructive.  As  Boveri  points  out,  it  shows  almost 
conclusively  that  the  pharynx  of  Amphioxus  does  not 
correspontl  to  the  pharynx  alone  of  the  higher  forms,  but 
to  the  pharynx  together  with  the  anterior  portion  of  the 
alimentary  canal. 

In  the  Craniota  the  gill-clefts,  which  are  present  in  a 
limited  number,  have  become  involved  in  the  complicated 


INTERNAL   A lY ATOMY. 


75 


I 


process  of  cephalisation,  by  which  the  Vertebrate  head  has 
been  evolved.  They  are  innervated  exclusively  by  the 
cranial  nerves,  and  in  fact  are  considered  as  forming  part 
of  the  head.  In  Amphioxus  there  is,  broadly  speaking,  no 
head,  and  the  region  of  the  gill-slits  forms  part  of  the  trunk. 
In  the  evolution  of  the  Craniota,  therefore,  what  has  hap- 
pened is  that  the  gill-clefts  have  been  relegated  to  the 
head,  while  the  excretory  tubules  have  become  confined  to 
the  trunk,  and  have  ceased  to  occur  in  the  neighbourhood 
of  the  gill-clefts.  Only  the  anterior  region  of  the  pharynx 
of  Amphioxus  is  represented  by  the  pharynx  of  the  higher 
forms.  The  greater  part  of  it  corresponds  to  the  unper- 
forated  portion  of  the  alimentary  canal,  which  follows 
immediately  behind  the  pharynx  in  these  forms,  extending 
to  the  liver. 

We  have  referred  above  to  the  absence  of  a  pronephric 
duct  in  Amphioxus.  Although  this  is  true  in  the  strict 
sense  of  the  term,  yet  Boveri  gives  reasons  for  supposing 
that  the  right  and  left  pronephric  ducts  are  in  a  measure 
represented  by  the  right  and  left  halves  of  the  atrial 
chamber.  (Cf.  Fig.  35,  A  and  />).  We  will  first  glance 
briefly  at  the  mode  of 


Development  of  the  Atrial  Cavity, 

For  the  sake  of  avoiding  complications,  it  will  be  well  to 
confine  the  description  at  present  to  the  mode  of  origin  of 
the  atrial  cavity  in  its  posterior  region  It  arises  of  course 
on  the  same  principle  throughout  its  whole  extent  (except 
the  post-at''ioporal  continuation,  which  grows  back  later) 
but  anteriorly  it  is  involved  in  the  asymmetry  which  is  such 
a  marked  feature  of  the  larva,  and  will  be  considered  in  the 
chapter  on  the  general  development. 

The  first  indication  of  the  future  atrial  cavity  appears  in 


76 


AXATOMV   OF  .IMI'I/IOXCS. 


ll 


u  V'uinif  larva  with  .some  six  or  seven  irill-slits  in  the  form 
oi  two  longitudinal  thickeninc^s  of  the  integument  on  the 
ventral  surface  of  the  body.  These  are  at  first  solid,  but 
eventually  become  hollowed  out  so  as  to  enclose  a  longitu- 
dinal canal  on  each  side.  This  is  the  so-called  metapleural 
canal  or  lymph-sj^ace.  The  thickenings  enlarge  to  the 
extent  of  forming  two  well-marked  folds  of  the  body-wall ; 
namely,  the  DictnpUnral folds. 

The  next  stage  is  marked  by  the  formation  of  two  small 
solid  longitudinal  ritlges  on  the  inner  opposed  faces  of  the 
metapleural   folds  (Fig.    36).      It    is    by    the   subsequent 


Figs.  36  and  37.  —  Schematic  transverse  sections  through  post-pharyngcal 
region,    iliiistrating   mode   of  origin  of  atrial  chamber.     (After    Lankesikr  and 

Wil.I.KV.) 

ao.  .-\oifa.  b.c.  Cn}lom.  r.vi  and  l.m.  Right  and  left  metapleural  fok.s.  s.a.?:  Sub- 
atrial  ridges,  which  fuse  together  to  form  the  floor  of  at,  the  atrium,  int.  Aliment- 
ary canal,    s.i.v.  Sub-intestinal  vein. 


meeting  and  coalescence  of  these  siibatrial  ridi^cs  that  the 
atrial  cavity  becomes  enclosed  as  a  small  median  tube  lined 
by  ectoderm. 

As  soon  as  it  has  become  closed  off  from  the  exterior, 
the  atrial  tube  commences  to  grow  in  size,  and  it  gradually 


fA'7'EK\A/.   AXA  TOMY. 


77 


expands  laterally  and  also  in  an  upward  direction,  propor- 
tionately reducinj;  the  extent  of  the  etxilom  as  it  tloes  so 
(Fi^-.  37;  cf.  also  Fig-.  26).  At  its  posterior  extremity  the 
atrial  tube  does  not  become  closed  in,  but  remains  perma- 
nently open  as  the  atriopore. 
It  is  a  curious  fact  that  the 
fusion  of  the  subatrial  rid<;es 
to  enclose  the  atrial  tube  takes 
place  j^radually  from  behind 
forwards,  so  that  f(jr  a  long 
time  the  latter  has  the  form 
of  a  canal  open  to  the  exterior 
at  both  ends.  The  chief  feat- 
ures in  the  formation  of  the 
atrium  are  shown  diagrammat- 
ically  in  Fig.  38,  A,  B,  and  C. 
In  Fig.  38  A  the  atrial  tube 
has  not  begun  to  be  closed  in, 
but  the  two  metapleural  folds 
are  seen  running  side  by  side 
for  some  distance.  Anteriorly 
the  development  of  the  right 
mctajileur  is  in  advance  of  that 


Fig.  38.  —  Three  plastic  dia^jrams 
of  larv:i;  of  .Amphioxus  from  the  ven- 


of    the    left,    and    it    is    seen    to    tral  aspect,  illustrating   tiie   nioilo  of 

.  .  enclosure  of  the  atrial  tube  from  be- 

bend    round    to    the    right    side    hind   forwards.     The   atrium    is   still 

of  the  body  in  correspondence  ^f'^^^,^^fo,ca   in    .i;    partially 

■^  '  closed  m  /> ;  and  almost  completely 

with  the  asymmetry  of  the  gill-  closed  in  c.    (After  lankkstkk  and 

1-  /        ■     7  -/-XTT  •  WlI.I.EY.) 

slits  {rtae  tiifra).  Having  ar-  p^.  Primary  giii-siits.  r.v,.  Right 
rived  at  the  front  end  of  the  "p'""'- AA '''•^^■orai  pit.  ..  Mouth. 

at. p.  Atriopore. 

pharynx,  the  right    metaplcur 

bends  sharply  inwards  to  tho  mid-ventral  line  and  then 
gradually  dies  out  in  front.  In  F^ig.  38  /?  the  subatrial 
ridges  have  met  and  fused  for  a  short  distance  behind  the 


wm 


7« 


ANATOMY  OF  AM  PHI  OX  US. 


pharynx,  so  as  to  enclose  a  tube  which  corresponds  to  that 
portion  of  the  future  atrial  cavity  which  lies  between  the 
atriopore  and  the  hinder  end  of  the  pharynx.  Finally, 
in  Fi<;.  38  C,  the  closure  of  the  atrial  tube  has  advanced 
forwards  over  the  gill-slits  almost  to  the  anterior  extremity 
of  the  i)harynx,  still  leavin-j:,  however,  one  or  two  j^ill-slits 
open  directly  to  the  exterior  in  front.  Meanwhile,  the 
floor  of  the  atrium  has  increased  in  width,  and  the  meta- 
pleural  folds  are  separated  by  a  wider  interval  than  before 
(Fig.  38  C).  I'vventually  the  atrium  closes  up  completely 
in  front,  so  that  the  gill-slits  no  longer  open  directly  to 
the  exterior. 

Remembering  that  the  atrium  of  .^Vmphioxus  arises  as  an 
unpaired  median  tube  (see  below,  I\'. ),  while  the  pro- 
nephric  duct  is  always  paired,  the  following  are  some  of 
the  reasons  for  supposing  a  partial  homology  between  the 
two  structures  :  — 

(a)  They  are  both  derived,  either  wholly  (atrium),  or  in 
a  large  measure  (proncphric  duct),  from  the  ectoderm.^ 
(yS)  They  both  receive  and  carry  away  the  excretory  i)rod- 
ucts  from  the  proncphric  tubules ;  and  (7),  they  are 
both,  to  a  greater  or  less  extent,  lined  by  an  epithelium, 
which  is  itself  glandular  and  excretory.' 

Covtparisixn  between  the  Excretory  System   of  Auiphioxns 
and  that  of  the  Aiuielids. 


Having  considered  the  relation  existing  between  the 
proncphric  system  of  Amphioxus  and  the  corresponding 
system  in  the  embryonic  and  larval  stages  of  the  higher 
Vertebrates,  we  will  now  pass  on  to  a  brief  comparison 
with  the  excretory  system  of  the  Invertebrates. 

The   excretory  system   of   a   typical   Annelid  presents 


/.V/7:A:\:I/.  AX.l  jomy. 


79 


certain  resemblances  to  that  of  Aniphioxus,  in  that  it 
occurs  in  the  form  of  distinct  scL;montal  tubules,  or 
mphridia,  each  possessing;  a  funnel-shapeti  opening  into 
the  l)ocly-cavity,  and  an  o[)enin<;  to  the  exterior  at  the  sur- 
face of  the  body. 

It  was,  in  fact,  the  recognition,  some  twenty  years  ago, 
by  Si;mi'K1<  and  Bai.iolk,  of  the  resemblance  between  the 
arrangement  of  the  nei)hri- 
(lia  of  the  Annelids  and 
the  jirimary  segmental  ori- 
gin of  the  kidney  of  the 
Craniota  that  was  chiefly 
instrumental  in  placing  the 
Annelid-theory  of  Verte- 
brate descent  on  a  tempo- 
rarily firm  basis. 

A  dissection  of  the  an- 
terior portion  of  the  body 
of  an  earthworm,  exposing       _,.  ...  ...       „ 

'        '            ^  Fig.   39. — .Anterior   portion    of   c;utli- 

the    nephridial     tubules,    is  worm  (lis>,citr(l  opi-n  from  above  to  sliow 

.                                             .  tlie  m-])lHi(lia  ;uul  nervous  svstcm.     (l'"ron» 

shown   \\\  iMg.  39.       A  pair  w.  T.  Si-dcwick   and   E.'  15.  Wilson's 

of  such  convoluted  tubules    <'^"^-''";/''"^".o;-) 

/;-.    Prostonnuin    (i)r;i.'oral    lobe),    c.i^. 

occurs  in  each  segment,  or  Ccrebralganglion.  which  has  receded  from 

.  the    prostomuim    from    the    ectoilerm    of 

ruig,     of      the      body,     com-  ^^^;\^\^\^    it    arose,     com.    (•ireum(i'So])h:i.i;c.d 

niencing    from      the     third,  commissure    surroundirig   the  buccal  tube 

"  (latter    not    represented).       v.n.c.   Ventral 

Physiologically,    of    course,  mrve-cord.      ;/.    Segmental    nerves,      /////. 

,                             ,•           1  N'(fphridia.    sp.  Dissepiments. 

they  are  directly  com- 
parable to  the  renal  tubules  of  the  Chordata,  and  in  their 
general  features,  allowing  for  the  absence  of  a  common 
duct,  the  similarity  in  the  two  cases  is  striking  enough. 
But  when  this  undoubted  similarity  is  used  as  an  argument 
for  deriving  the  Vertebrate  excretory  system  directly  from 
that  of  the  Annelids,  we  tread  on  very  uncertain  ground. 


It 


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IMAGE  EVALUATION 
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Photographic 

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Corporation 


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23  WEST  MAIN  STREET 

WEBSTER,  N.Y.  MS80 

(716)  872-4S03 


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'O 


AXATOMY  OF  AMPIIIOXL'S. 


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iti'll  f 
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II* 


If  we  were  to  consider  the  excretory  system  apart  from 
the  rest  of  the  oriranisation,  this  would  be  the  onlv  course 
to  follow.  But  when  the  whole  organisation  is  taken  into 
account,  the  only  justifiable  conclusion  seems  to  be,  not 
that  the  Vertebrate  renal  system  is  to  be  derived  from  that 
of  the  Annelids,  but  that,  as  Riickert  suggests,  both  may 
possibly  have  been  evolved  from  a  common  starting-point. 

It  is  eminently  probable  that,  in  respect  to  this  and  the 
other  systems  of  organs,  as  well  as  the  segmentation  of 
the  body,  the  Annelids  and  Vertebrates  present  an  in- 
stance of  parallel  ivolittion.  This  will  become  more  evi- 
dent as  we  proceed.  Those  who  uphold  the  so-called 
Annelid-theory  have  no  cause  to  complain  of  the  absence 
of  a  common  duct  to  the  nephridia,  since  this  has  been 
found  in  some  cases  to  occur. 

In  1884  EnuARD  Meyer  discovered  that  in  certain 
marine  Annelids  {Laiiicc  conchilc^a  and  Loimia  medusa) 
belonging  to  the  family  of  the  Terebellida},  the  nephridia 
of  each  side  were  joined  together  by  longitudinal  ducts, 
which  he  compared,  though  with  great  reserve,  to  the 
mesonephric  ducts  of  the  Vertebrata.*  In  these  worms  the 
nephridia  do  not  occur  in  all  the  segments  of  the  body,  but 
are  confined  to  the  anterior  so-called  thoracic  region,  their 
number  being  very  limited.  In  the  thora.x,  the  dissepi- 
ments which  typically  divide  the  segments  from  one 
another  are  absent,  so  that  the  body-cavity  would  here 
form  a  continuous  uninterrupted  space,  were  it  not  that  it 
is  divided  into  two  chambers,  an  anterior  and  a  posterior, 
of  which  the  latter  is  the  larger,  by  a  muscular  diaphragm. 
In  the  anterior  thoracic  chamber  (Fig.  40)  there  are  three 
pairs  of  nephridia  which  are  united  together  on  each  side 
by  a  short  duct  opening  to  the  exterior  by  a  single  aperture. 

♦  This  discovery  was  also  made  later  hut  independently  hy  J.  T.  CfNNiNi;- 
HAM  for  J.aiiiie  lOHc/iiltxa. 


IXTEKXAL  ANA  TOMY. 


8l 


In  the  posterior  chamber  there  are  four  pairs  of  much 
larger  ncphridia,  which  are  similarly  joined  together  by  a 
prominent  longitudinal  duct  from  which  short  processes 
corresponding  in  number  to  the  nephridia  lead  to  the 
external  apertures.  The 
duct  itself  ends  blindly  at 
both  ends,  but  is  prolonged 
jxisteriorly  far  beyond  the 
region  of  the  nephridia 
(Fig.  40). 

The  presence  of  this 
longitudinal  duct  in  these 
worms  is  a  very  remark- 
able circumstance,  but  it  is 
undoubtedly  an  expression 
of  the  same  phenomenon  as 
the  anastomc  es  between 
successive  nephridia  which 
have  been  described  by 
EisiG  for  the  Capitellidae, 
as  well  as  the  complicated 

series  of  anastomoses  which  Pig.  40.  —  Schematic    lateral    view   of 

.      anterior  end  of  Lattice  coticliilc^a  to  stimv 

convert  the  entn-e  nephri-  the  nephridia.    (After  eduari.  mf.yiu 
dial  system  into  a  marvel-  fr"niHatsehek-s /M;-W;,,  ./.,/../,.;v,.^ 

■'  I  lie  ventral  side  ot  the  Ixidv  is  to  the 

loUS  network  of  tubules  dis-  lt>ft  of  the  fisuie.     d.  F.ongitiKlinal  ducts  of 

A      /-^     13  '''^'    "•^'Pli'"''!''^-      ''•^'    Position    of    external 

covered   by   A.    G.   IiOUKNE  openings.    /C  X.-phridial  funnel  (-e.rlomic 

in    the    marine    leech,    Pan-    opening  of  nepl;ridium).     w.    Position   of 

mouth;  bountled  by  two  prominent  lateral 

tobch'lldy   and    by   BeDDARD    lobes,  and   fringed   by  a  great    number  of 

,  .  ,  "  feelers,"  which  are  cut  short  in  the  figure. 

m  the   curious    earthworm,    /.  Branchial  tentacles   (three  on  each  side 

Pcrichceta.  of  the  body). 

The  present  state  of  our  knowledge  docs  not  admit  of 
an  attempt  to  specify  the  particular  type  of  nephridial 
sy.stem  from  which  that  of  the  Annelids,  on  the  one  hand, 


I 


M 


82 


ANATOMY  OF  AMPHIOXUS. 


and   that    of   the   Vertebrates,   on   the   other,  took    their 


origin. 


1! 


H 


In  view  of  the  apparent  absence  of  nephridial  tubules 
in  Bahnoglossus  and  the  fact  that  in  the  Ascidians  the 
renal  organs  arc  special  structures  peculiar  to  this  group, 
it  is  extremely  difficult  to  associate  the  Vertebrate  type 
of  excretory  system  with  that  of  any  Invertebrate. 

Since  the  Annelid-theory  precludes  the  possibility  of 
Amphioxus  being  regarded  as  an  ancestral  form,  and  yet 
if,  nevertheless,  it  is,  as  we  believe,  primitive  and  not 
essentially  degenerate,  the  discovery  of  the  excretorv 
tubules  in  Amphioxus  happily  releases  us  not  only  from 
necessity,  but  also  from  the  possibility  of  referring  the 
Vertebrate  excretory  system  back  to  that  of  the  Annelids. 

NerooHs  System. 

The  central  nervous  system  of  Amphioxus  consists  of  a 
closed  thick-walled  tube  lying  along  the  dorsal  side  of  the 
body  above  the  notochord. 

Viewed  externally,  it  is  a  perfectly  plain,  more  or  less 
cylinder-shaped  structure,  without  any  constrictions  or 
enlargements  whatever.  Its  largest  diameter  in  the  adult 
occurs  about  the  middle  of  its  course,  and  not  at  its 
anterior  end. 

Posteriorly  it  is  nearly  coextensive  with  the  notochord, 
and,  like  it,  tapers  down  almost  to  a  point.*  Anteriorly  it 
terminates  abruptly  some  distance  behind  the  front  end 
of  the  notochord.     (Cf.  Figs.  3  and  11.) 

If  the  dorsal  nerve-cord  be  removed  from  the  body  and 

♦  The  extreme  posterior  end  of  the  nerve-cord  is  usually  swollen  out  into 
a  small  ampulla-like  dilatation.  (PorciiKT,  RoHON,  Retzhs.)  Ri  rzii  s 
has  observtd  that  occasionady  the  nerve-cord  is  prolonged  beyond  the  dilata- 
tion and  actually  bends  round  the  posterior  end  of  the  notochord. 


IXTEKNAL   AXA  TOM ) : 


8- 


^  r 


examined  from  above,  its  general  appearance  will  be  as 
shown  in  Fig.  41.  In  front  there  is  a  pair  of  nerves 
which  proceed  symmetrically  from  the  sides  of  the  nerve- 
tube.  Farther  back  there  is 
another  pair  of  nerves  which 
arise  more  dorsally  than  the 
anterior  pair,  but  are  likewise 
placed  symmetrically  one  oppo- 
site the  other.  Hehind  this 
second  pair  of  nerves  the  spinal 
nerve-roots  are  no  loniicr  dis- 
posed  symmetrically,  but  alter- 
nate with  one  another,  in  cor- 
respondence with  a  similar 
alternation  of  the  myotomes, 
the  alternation  becoming  more 
and  more  pronounced  as  we 
proceed  backwards.  Again,  be- 
hind the  second  pair  of  nerves 
there  are  two  kinds  of  spinal 
nerve-roots,  dorsal  and  ventral. 
The  former  leave  the  nerve-cord 
from  its  dorsal  surface,  and  the 

1    ^.         r  ii-  •  r    ■..  Pig.   41.  —  Anterior  portion   of 

latter    from    the    margms    of    its    ^pinul   cord   of   Amphioxus;    seen 

ventral  side.     In  the  dorsal  roots   ^""o'"  -"ihove.    (After  schnkidf.k.) 

Between  the  first  pair  of  cranial 
the  nerve-fibrils  are  collected  nerves  is  seen  the  eye-spot ;  one  of 
.  .  V         J.       r  •       1  the  branches  of  the  second  pair  of 

too'ctner  to  lorm  a  single  com-         ■  ,  •  • 

'.,vLuv.i    \.vt   iwiiii    ii    .Tui.^n.    v-v^in      cranial     nerves    sometimes    arises 

pact      nerve      round     which     the    tlircctly   from    the    spinal    cord   as 

shown   on   the   right;    farther  back 
sheath  of    the  nerve-cord  is  con-    are  seen  the  pigment  spots  of  the 

tinned,  while  in  the  ventral  roots   "^'•"'''^^-""■''• 
the  nerve-fibres  emerge  separately  in  loose  bundles  unsur- 
rounded  by  a  sheath,   from  the   spinal   cord.      A  pair  of 
dorsal  roots  and  a  pair  of  ventral  root-bundles  go  to  each 


pill 


84 


AXATOMY   OF  AMJ'J/IOXUii. 


segment  of  the  body.  Dorsal  and  ventral  roots  are  entirely 
independent  of  one  anotln;r,  and  at  no  point  do  they  coa- 
lesce as  they  do  in  the  Craniota.     In  further  contrast  to 


Un 


Fig.  42  A.  —  Innervation  of  the  region  of  the  oral  hood  and  snout.  (After 
Haischek,  slightly  altered  according  to  the  statements  of  \'.\N  WljHK.) 

c/i.  Anterior  end  of  notochord.  ci.  Buccal  cirri,  cn^,  en'-.  First  and  second 
cranial  nerves  with  their  peripheral  ganglia,  cu.  Rami  cutanei  dorsales.  /.h.  Left 
half  of  oral  liood.  r./t.  Right  half  of  oral  hood.  o.  Olfactory  pit.  .?/',  sp-.  First 
and  second  dorsal  spinal  nerves,  so.  Sense-organ  of  oral  hood  (groove  of  Hat- 
sclu'k)  indicated  as  if  seen  through  body-well  by  transparency,  v.  X'eluni. 
z\ii./.  .Verve  to  left  side  of  velum,    v.n.r.  Nerve  to  right  side  of  velum. 

N.15.  —  The  septa  between  the  myotomes  art  indicated  by  dotted  lines.  The 
superficial  nerves  of  oral  hood  arc  rendered  in  black;  the  deeper  nerves,  which 
anastomose  to  form  the  plexus  of  Fusari,  are  left  white. 


the  condition  met  with  in  the  latter  there  is  no  ganglionic 
enlargement  on  the  dorsal  root. 


m'.-MB 


cono 
Left 
First 
Hat- 

■luiii. 

Tin- 
vliicli 


LXTEKAAL  ANA  JOMY. 


85 


The  first  two  pairs  of  nerves  differ  in  many  points  from 
those  which  succeed  them,  and  are  known  as  the  cranial 
iicn'cs.  Thus  they  have  no  corresponding  ventral  roots  ; 
they  appear  to  be  exclusively  sensory,  and  do  not  inner- 
vate any  muscles ;  their  distribution  is  confined  to  the 
snout,  and  they  are  above  all  characterised  by  the  pres- 
ence of  peripheral  ganglionic  enlargements  which  occur 
chiefly  on  the  finer  branches  of 
the  nerves  near  their  distal  ex- 
tremities. Furthermore  they  lie 
in  front  of  the  first  myotome. 
The  first  pair  of  dorsal  spinal 
nerves  {i.e.  the  third  pair  alto- 
gether) belonging  to  the  first 
myotome  passes  from  the  nerve- 
tube  to  the  skin  through  the 
dissepiment  which  separates  the 

Fig.  42  B.  —  Diagram  illustrat- 
first    myotome    from  the  second,     ingthebr.mcliinyof  a  <U)is.il  .spinal 

Ai  -i-u       11    ..u  !•  nerve  of  Amiihioxus.     (After  Ha  1- 

nd  so  with  all  the  succeedmg  ^,.,,.,,.  > 

dorsal  roots,  they  lie  at  the  back       •''■':  i^o^sai  root.    /•.</.  Ramus 

dorsalis.  r.v.  Ramus  ventr.ilis. 
of  the  myotome  to  which  they  r.vi.  Ramus  visceralis.  r.c.  Ramus 
11       „!«.  -4.  i«.u„^t    cutaneus  ventralis  innervating  ecfo- 

beiong,  between  it  and  the  next   ^^^^^  ^,^  n,e,apieur.    -....  Vent.ai 

following    segment.        (Cf.     Figs,     or  motor  root,  indicated  as  if  in  tlie 

same  plane  as  the  dorsal  root. 

2  and  42  A.) 

Shortly  after  leaving  the  central  nervovs  system,  the 
dorsal  roots  divide  into  two  branches,  a  ranuis  dorsalis 
and  a  ramus  ventralis  (Fig.  42).  These  two  branches  run 
upwards  and  downwards  respectively,  in  the  gelatinous 
layer  of  the  sub-epidermic  cutis  ;  that  is  to  say,  external 
to  the  muscles. 

In  the  Craniota  the  corresponding  branches  of  the 
spinal  nerves  lie  for  the  first  part  of  their  course  internal 
to  the  muscles,  between  the  latter  and  the  notochord.     The 


t.ifll-| 


v^i 


41    '...15  !  'VS'f  « < 


,1    I 


;ri.'- 


ifF^ 


86 


ANATOMY  Of  AMP/flOXi'S. 


cranial  nerves  of  the  Craniota  so  far  resemble  the  dorsal 
spinal  nerves  of  Amphioxus  that  they  run  external  to  or 
ectad  of  the  somites  of  the  head. 

The  ramus  dorsalis  of  a  spinal  nerve  breaks  up  into  a 
number  of  finer  nerves,  which  supply  the  skin  of  the  back. 
The  ramus  ventralis  similarly  i;ives  lise  to  a  number  of 
cutaneous  nerves,  but  in  addition  it  t;ives  off  a  branch 
which  passes  inwards  below  the  longitudinal  muscles  of 
the  body-wall,  between  them  and  the  transverse  muscles 
which  lif'  in  the  floor  of  the  atrium.  This  is  the  visceral 
branch  of  the  spinal  nerve.  The  visceral  nerves  innervate 
the  transverse  muscles  and  form  an  elaborate  plexus  on 
the  surface  of  them.* 

Thus  the  dorsal  spinal  nerves  of  Amphioxus  are  of  a 
mixed  nature,  sensory  and  motor,  but  chiefly  sensory. 

The  ventral  roots  are  entirely  motor.  On  their  emer- 
gence from  the  spinal  cord  they  spread  out  like  a  fan 
and    terminate  upon   the  muscle-fibres  of  the  myotomes 

The  muscles  which  are  not  innervated  by  the  ventral 
spinal  nerves  are  the  transverse  or  subatrial  muscles,  the 
muscles  of  the  month  (velum),  and  oral  liooiL  and  probably 
the  anal  sphincter.  These  are  supplied  by  the  so-called 
visceral  branches  of  the  dorsal  nerves.  The  nerve-supplv 
of  the  oral  hood  is  illustrated  in  Fig.  42.  It  arises  from 
branches  of  the  third  to  the  seventh  dorsal  nerves.  These 
branches  are  distributed  in  two  different  ways  :  one  set 


i' 


*  The  visceral  nerves  also  send  up  branches,  which  ]iass  up  thri)ugli  tlie 
ligamentuni  denticulatuni  to  the  wall  of  the  pharynx.  (Kr.sAUi;  see  below, 
p.  .)  Here  they  form  the  l)ranchial  plexus  descrilied  by  KoUuN,  who 
thought  these  nerves  contained  elements  of  the  I'a^^us  of  the  Craniota. 
The  portions  of  the  visceral  nerves  innervating  the  transverse  muscles  (these 
branches  being  discovered  by  Roi.ril)  were  held  by  RoikiN  to  contain 
elements  of  the  Sympatlu'tic  system  of  Craniota. 


>■■■ 

I 


IXTKKX.l/.   AA'A  TOMY. 


87 


of  them  runs  beneath  the  outer  surface  of  the  oral  hood 
ami,  by  the  occurrence  ot  frequent  anastomoses,  forms  a 
coarse  network  known  as  the  outer  plexus,  while  the  other 
set  lies  beneath  the  inner  surface  of  the  oral  hood  and 
^ives  rise  to  the  iniur  plexus.  The  latter  was  discovereil 
hv  FusARi  in  1S89.     The  two  plexuses  are  distinct  from 


Fig.  43- — Transverse  section  through  the  spinal  cord  in  the  middle  region  of 
tht"  l)0(ly.     (After  KoilDi:.  I 

a.  (iiiint  fibre  j^roceeding  from  the  giant  ganglidn-cell  ./  (see  l)elo\v).  c.c.  Cen- 
tral canal.  ,4'. /"'.  (jiant  nerve-fibres,  which  traverse  the  spinal  cord  from  bi'fore 
l)ai.'kvvar(ls.  ,;■•./'-.  Cjiant  fibres,  which  traverse  the  spinal  cord  from  behind  for- 
w.inls.  in. p.  .Muscle-]ilates.  tn.r.  Motor  nerve-fibres,  it.f.  Longitudinal  nerve- 
t'llirt's  cut  across,    s./.  Supporting  fibres,    sh.  Sheath  of  nerve-cord  (=  dura  mater  ; 

l'l>ARI). 

one  another,  except  in  so  far  as  their  component  nerves 
have  a  common  oriy;in  from  the  dorsal  roots  (Fig.  42). 
The  outer  plexus  is  continued  up  into  the  individual  cirri, 
while  the  inner  plexus  appears  to  stop  short  at  the  base 
of   the   cirri.     It    has   recently  been    discovered   by  van 


88 


ANATOMY  OF  A  MP/// O  XL'S. 


U'|ii>- 


WijHE  that  the  inner  plexus  on  both  riy;ht  and  left  halves 
of  the  oral  hood  is  exclusively  formed  by  nerves  which 
arise  from  the  /eft  side  of  the  central  nervous  system  ; 
and,  further,  that  the  nerve-supply  of  the  velum  is  fur- 
nished by  branches  from  the  fourth,  fifth,  and  sixth  dorsal 
nerves  of  the  U  ft  side  only.  This  asymmetrical  innerva- 
tion ol    the  velum    and    inner  (<,dandular)  surface  of  the 

oral  hood  will  be  referred  to 
again  after  the  consideration 
of  the  larval  development. 

The  peripheral  ganglionic 
enlargements  which  are  so 
characteristic  of  the  two  pairs 
of  cranial  nerves  must  be  cor- 
related with  the  sensibility  of 
the  snout.  As  the  nerve-fibres 
are  continued  beyond  them, 
they  are  not  to  be  regarded  as 
end-organs,  but  simply  as  peri- 
Fig.  44.  -  ivriphL-rai  sangiion-  phcral  ganglia.  Their  structure 

cells  of  the  cranial  nerves  of  Amplii-  jg  shown  in  Fi""  AA.  T1'"V 
oxus.     (After  l-TSARl.)  '^^     ^^'  '  '■' 

were  discovered  by  the  great 
French  naturalist  Ouatrefages  in  1845.  Each  of  them 
is  composed  of  from  one  to  four  nerve-cells,  with  granular 
protoplasm  and  a  large  nucleus.  Each  grouj)  is  enclosed 
in  a  sheath  which  is  a  continuation  of  the  sheath  of  the 
nerve  itself.  The  sheath  is  lined  internally  by  an  erdo- 
thelium.  According  to  Fusari  the  nerve-fibres  enter  into 
direct  connexion  with  the  cells,  though  some  would  appear 
to  pass  round  them. 

The  peripheral  nervous  system  of  Amphioxus  can  only 
be  compared  definitely,  at  present,  in  its  broadest  features 
with  that  of  the  higher  Vertebrates.     The  determination 


ii 


/.\- TliRXAL   ANA  TOM  \ \ 


89 


of  the  particular  homoloo;ics  in  the  two  cases  forms  one  of 
the  most  difficult  problems  of  comparative  morpholo;;)-.  In 
correlacion  with  the  low  grade  of  cephalisation  to  which 
Amphioxus  has  attained,  there  are  only  two  j>airs  of 
cranial  nerves,  the  succeedin<;  nerves  retaining  their 
primitive  spinal  character.  The  dorsal  spinal  nerves, 
furthermore,  possess  features  which  are  specially  charac- 
teristic of  the  cranial  nerves  of  the  Craniota.  Such  are 
their  mixed  sensory  and  motor  functions,  and  the  position 
of  their  dorsal  and  ventral  branches  ectad  of  the  muscula- 
ture. As  already  indicated  above,  the  walls  of  the  gill-slits 
of  the  craniate  Vertebrates  are  innervated  by  cranial 
nerves,  while  in  Amphioxus  this  is  done  by  spinal  nerves. 
(Cf.  P'ig.  92  ;  see  also  below,  p.  163.) 

In  transverse  section  the  spinal  cord  of  Amphioxus  is 
seen  to  have  somewhat  of  a  triangular  shape.  The  central 
canal  has  the  form  of  a  vertically  elongated  split,  commenc- 
ing from  the  vertex  of  the  triangle,  and  extending  two- 
thirds  of  the  way  downwards  into  the  cord.  For  the 
greater  part  of  its  extent,  however,  the  two  sides  of  the 
canal  are  closely  approximated  together  so  as  to  obliterate 
the  lumen,  which  widens  out  again  below,  and  presents  the 
appearance  of  a  circular  or  oval  tube.  The  sides  of  the 
canal  are  lined  by  an  epithelium  the  cells  of  which,  starting 
from  an  indifferent  condition  in  the  embryo,  have  become 
modified  in  several  different  directions.  Some  vccfi gmigliivi- 
cells,  and  others  send  out  long  radial  processes  which  trav- 
erse the  substance  of  the  nerve-cord,  and  serve  to  hold  it 
together.  These  are  the  snp/^orfing  fibres  (Fig.  43).  The 
cells  in  the  nerve-cord  form  a  much  smaller  proportion  of 
the  bulk  of  it  than  the  nerve-fibres  do.  The  latter  run 
mostly  in  a  longitudinal  direction,  and  produce  a  punctate 
appearance  in  cross-section. 


90 


AXATOMV   or  AM/'I/IOXCS. 


Anteriorly  in  the  region  of  the  cranial  nerves  the  lumen 
of  the  central  canal  widens  out  into  a  relatively  spacious 
vesicle,  known  as  the  cerebral  vesicle  (Vv^.  45).  In  youn^ 
individuals  this  cavity  opens  by  an  ajjerture  called  the 
nciiroporc  into  the  base  of  an  epidermal  pit,  which  we 
have  already  described  under  the  name  of  the  olfactory 
pit.     Later  on    the   neuropore  closes  up,   but   its  former 


,!   '     ' 


t-         1 


it 


ii 


Fig.  45.  — ./.  Mrain  and  cranial  nerves  ot  a  young  Aniphioxus  of  3  mm.  length. 
n,  (',  />.  Sections  tlirdu^h  clitVerent  poitions  ot  biain:  />',  throiigli  neuropore  and 
cercbtal  vehicle ,  ^',  tlirougii  the  interineiliate  jioition,  .n  id  /'.through  the  dorsal 
dilatation  of  central  canal.     (After  Ha'ISCUKK. ) 

i/i.  Notuehord.  i.v.  Cerebral  vesicle.  <///.  Dors.il  dilatation  (Hatsciiek's  K's.-.i 
I /ioi)ilH>ii/alis).     t'.  Eve-spot.     ///.Neuropore.     <'//;  Olfactory  pit. 

/,  //.  l-'irst  and  second  cranial  nerves. 

jiresence  is  indicated  by  a  shallow  tjroove  at  the  base  of 
the  otherwise  solid  stalk  connecting  the  olfactory  pit  with 
the  roof  of  the  brain. 

Behind  the  cerebral  vesicle  the  lumen  of  the  central 
canal  widens  out   in   its  dorsal   portion   independently  of 


i^;- 

n 

1    .    , 

LVTEKXAI.   AAA  T(  >M  1 '. 


9« 


the  ventral  tube,  so  as  to  form  a  vesicular  (lilatatu)u  cov- 
ered over  by  a  thin  membrane.  The  rej^ion  of  the  nerve- 
tube,  over  which  this  dorsal  dilatation  extends,  has  been 
compared  by  IIatscmek,  who  discovered  it,  to  the  iiiidit/ld 
vMoNi^ata  of  the  craniate  Vertebra  whi(.h  is  similarly 
roofed  in  only  by  membrane.  In  the  fijUv  i^rown  condi- 
tion, however,  it  seems  to  be   largely  obliterated  by  the 


||#|iw 


Fig.  46.  — 'I'lanhViisf  s(  ction  ihr(iiii;li  tin-  ^^llin.ll  cord  hefwecn  the  second  ;inil 
tliird  .sensory  roots.     (.Atti'r  Kiilinr,.) 

.i,'.*'.  l)orsal  afijjri'ijation  ot  ij.inglion-ci'lls  (extending  between  the  seeoiid  anci 
filth  pairs  of  sensory  nerves;  a  somewhat  siniihir  Kioiip  of  ijanghon-cells  oeeiirs  (Jii 
ventral  side  of  nerve-cord  below  the  central  canal  between  the  fourth  and  sixth 
sensory  nerves.) 

</./■.  Dorsal  root.  ,f./ Su])portin.<;  fibres,  f.f.  cenirui  tan.il ;  in  this  case  efjuaily 
w  ide  throughout  its  entire  iicight,  and  so  all  along  the  spinal  cord.  sh.  Sheath  of 
nerve-cord. 

development  of  a  mass  of  larj^e  j,Mnglion-cells  which  e.x- 
tend  backwards  as  far  as  the  fifth  pair  of  sensory  nerves 

(Fit;--  46). 

All  there  is  of  a  brain  in  Amphioxus  is  shown  in  Fit;. 
45.  The  cerebral  vesicle  is  a  plain  cavity  without  any 
true  subdivision  into  ventricles."     In  the  development  of 


\m 


WW 


!f 


li 


ir 


92 


AxVATOMY   OF  AMPHIOXUS. 


the  central  nervous  system  of  the  higher  Vertebrates,  a 
stage  is  passed  through  which  may  be  compared  broadly 
with  the  permanent  condition  of  things  in  Amphioxus. 
But  in  the  former  the  anterior  portion  of  the  medullary 
tube  quickly  becomes  greatly  enlarged  in  contrast  to  the 
spinal  cord  proper,  and  becomes  divided  by  constrictions 
into  fore-,  mid-,  and  hind-brain,  which  constitute  the  three 
primary  divisions  of  the  Vertebrate  brain.  Then  the 
brain  undergoes  a  flexure  round  the  anterior  end  of  the 
notochord.  This  curvature  of  the  primitively  horizontal 
brain-region  in  the  craniate  Vertebrates  is  known  as  the 
cranial fltxiin.     (Cf.  Figs.  23  and  24.) 

Among  the   numerous  longitudinal    nerve-fibres  which 
compose  the  bulk  of  the  spinal  cord  of  Amphioxus,  there 

are  some  which  stand  out  in 
marked  contrast  to  the  great 
-G  majority  on  account  of  their 
large  size.  These  are  the  so- 
called  giant-fibres,  and  they  form 
one  of  the  greatest  peculiarities 
in  the  spinal  cord  of  Amphioxus. 
According  to  Rohdk  there  are 
no  fewer  than  twentv-six  of  these 
giant-fibres  present,  and  each  of 

Fig.    47.  —  Transvi'isc    section 

throiiKii    spinal    cord    in    region  them  ariscs  from  a  corresjiond- 

of    Ri.u,t    «anglion-cell     G.     (After    •  -^^    o;an<rlion-CclL       TheSC 

a.  I'roeess  of  giant-cull  ,/.  ^i^.f.  so-callcd  giaut-cclls  havc  many 

Giant-t'ibres.  . 

processes,  i.e.  they  a'"e  multi- 
polar, but  they  each  send  out  one  main  stem,  which  is  a 
friant-fibre.  The  giant-cells  lie  across  the  middle  of  the 
central  canal,  and  the  giant-fibres  pass  outwards  alter- 
nately to  the  right  or  left  of  the  central  canal,  and  then 
bend  downwards  and  pass  below  the  central  canal  and  up 


II 


/--/• 


INTERNAL   ANATOMY.  93 

to  the  opposite  side  of  the  canal,  where 
they  continue  their  course  in  the  longitu- 
dinal direction  (Fig.  47).  The  giant-fibre 
belonging  to  the  most  anterior  giant-cell 
differs  in  several  respjcts  from  the  other 
jriant-fibres.  It  is  much  larger  than  the 
others,  and,  whereas  the  latter  lie  on  either 
side  of  the  nerve-cord,  the  fibre  in  question  51;  :.,•>::;.;;.. 
lies  in  the  middle  line  immediately  below 
the  central  canal  (Figs.  43  and  47). 

These  giant-fibres  traverse  the  spinal 
cord  almost  throughout  its  entire  length, 
stopping  short  at  som«'  distance  from  its 
anterior  and  posterior  ends.  The  giant- 
cells  are  arranged  one  after  the  other  in 
two  groups,  one  group  lying  in  the  anterior 
third  of  the  spinal  cord,  the  fibres  from 
which  run  backwards,  and  the  other  group 
occupying  the  posterior  third  of  the  cord, 
the  fibres  from  which  run  forwards  (Fig. 
48). 

The  giant-fibres  are  in  no  direct  con- 
nexion with  the  outgoing  nerves,  but  the 
giant-cells  usually  occur  opposite  a  sensory 
{i.e.  dorsal)  root  (Fig.  49). 

In  the  spinal  cord  of  Petromyzon  giant- 
fibres  are  present  in  considerable  numbers. 


Fig.  48.  —  Scheme  illustrating  the  course  of  the  giant- 
fibri's  and  their  origin  from  the  giant-cells  ./-/  in  the  spinal 
cord  of  Amphio.\us.     (After  RuUDE.)  ^'g-  48. 

./-/..  Giant  ganglion-cells  whose  giant  processes  traverse  the  spinal  cord  from 
before  i)ack\vards.  A  is  about  at  the  level  of  the  sixth  sensory  root,  counting  tiom 
the  first  cranial  nerve.  M-'/..  dant  ganglion-cells  wliose  giant  processes  traverse 
thi*  spinal  cord  from  behind  forwards.  M  is  about  at  the  level  of  the  fortiet';  sen- 
sory root. 


■i4^ 


W 
« 


iJ. 


94 


ANATOMY  OF  AM  PHI  OX  US. 


while  in  the  higher  Fishes  and  tailed  Amphibia,  as  well 
as  in  the  tadpoles  of  the  anourous  Amphibia,  the  giant- 
fibres  are  represented  by  the  so-called  fibres  of  Mauthncr* 

They  are  not  found  in  the  spinal  cord  of  adult  tailless 
Amphibia,  Birds,  and  Mammals.^^ 

Their  occurrence  in  such  large  numbers  in  Amphioxus 
is  therefore  the  symbol  of  an  archaic  organisation. 

Giant-fibres  form  a  very  striking  feature  in  the  ventral 
nerve-cord  of  many  Invertebrates.     Here,  however,  they 


"It 


■■■mo 


Fig.  49. --Part  of  spinal  c-^rd  seen  from  above;  from  a  preparation  stained 
with  methylene-blue.     (After  Retzius.) 

g.c.  Giant  ganglion-cell  lying  across  central  canal,  mo.  Motor  root,  s.  Sensory 
root. 

appear  often  to  lose  their  nervous  function,  and  serve 
rather  as  elastic  supporting  rods  for  the  nerve-cord.  They 
are  enclosed  in  thick  sheaths  of  connective  tissue,  and 
have  been  found  to  originate  in  giant  ganglion-cells. 
When  the  enclosed  nerve  degenerates,  they  become  hol- 
low tubes  containing  a  coagulable  fluid.     (Eisig.) 

With  regard  to  the  internal  origin  of  the  nerves  which 
pass  out  from  the  spinal  cord,  our  knowledge  only  extends 
to  the  dorsal  roots.     At  the  base  of  the  ventral  roots  the 

*  Also  known  as  Miillerian  fibres. 


!. 
ii 


irli! 


LVl^ERXAL  AXATOMY. 


95 


fibres  appear  to  stop,  and  in  their  place  a  peculiar  granular 
structure  of  unknown  significance  is  found  (Fig.  49). 

The  fibres  which  constitute  a  dorsal  root  are  derived 
from  two  sources.  Part  01  them  are  continuations  or 
branches  of  the  longiiudinal  fibres  on  the  same  side  of  the 
nerve-cord,  on  which  a  gi"/en  dorsal  root  may  be,  while  the 
other  moiety  appears  to  arise  largely  from  groups  of  small 
bipolar  ganglion-cells  in  the  neighbourhood  of  the  central 


Fig.  50.  —  Diagram  illustrating  the  internal  origin  of  the  nerve-fibres  of  a  sen- 
sory root.     (Combination  of  two  figures  of  Rktzil'S.) 

The  cells  giving  rise  to  the  processes  lying  on  the  same  side  as  a  sensory  root 
A,  which  divide  into  a  T  at  the  base  of  the  root,  are  naturally  in  contiguity  with 
the  central  canal,  but  are  displaced  for  the  purpose  of  the  diagram,  m.i.  Middle 
line, 

cnal,  which  send  one  process  each  in  the  direction  of  the 
dorsal  root,  and  another  process  from  the  opposite  pole  of 
the  cell  to  join  in  with  the  longitudinal  fibres  of  the  other 
side  of  the  spinal  cord  (Fig.  50).^^ 

We  will  now  compare,  or  rather  contrast,  the  central 
nervous  system  of  Amphioxus  with  that  of  an  Annelid 
such  as  the  common  earthworm.  The  type  of  nervous 
system  presented  by  the  latter  is  common  to  a  vast  propor- 
tion of  the  Invertebrates.     It  consists  essentially  of  three 


^i 


m4 


Hiif    ! 


96 


ANA  TOM  y  Of  AM  PHI  OX  US. 


very  sharply  defined  parts  (Fig.  39) ;  namely,  (i.)  the  cerebral 
or  supraccsopJiageal  gan<.,li()n,  which  is  situated  dorsally 
over  the  buccal  cavity;  (ii. )a  longitudinal  solid  nerx'c-cord 
composed  of  two  more  or  less  distinct  halves,  running 
along  the  whole  length  of  the  ventral  side  of  the  body 
below  the  alimentary  canal  ;  (iii.)  the  circumccsophagcal 
nerve-ring  or  eonnnissnre  which  encircles  the  buccal  tube 
and  ct)nnects  the  cerebral  ganglion  with  the  subccsopliageal 
ganglion  at  the  anterior  extremity  of  th'e  ventral  nerve- 
cord. 

X'^iewed  from  above  (as  in  Fig.  39),  the  ventral  nerve- 
cord  presents  a  series  of  constrictions  which  are  in  some 
forms  very  pronounced.  The  w'der  portions  occur  in  the 
middle  of  the  body -segments,  and  constitute  the  ventral 
ganglia,  which  are  strung  together  by  the  intervening 
nerves  (connectives)  in  the  form  of  a  ganglionic  chain. 
From  the  ganglia,  paired  nerves  pass  out  to  the  organs  of 
the  body. 

One  of  the  greatest  peculiarities  in  the  type  of  nervous 
system  abo^e  described  lies  in  the  fact  that  the  alimentary 
canal  passes  through  and  is  surrounded  by  a  portion  of 
the  central  nervous  system  ;  namely,  thecircumoesophageal 
commissure.  This  fact  has  been  one  of  the  most  serious 
difificulties  which  the  upholders  of  the  Annelid-theory  have 
hatl  to  contend  with. 

In  the  Chordata  the  alimentary  canal  does  not  pierce 
the  central  nervous  system  in  any  sense  whatever.*  Never- 
theless, there  have  been  many  conjectures  as  to  a  possible 
equivalent  of  the  circumoesophageal  nerve-collar  in  the 
Vertebrates,  although  it  is  safe  to  say  that  nothing  of  the 
kind  reallv  exists. 


*  Ba'.anoglossus  might  be  iaid  to  uffer  an  exception   to   this   rule    'see 
Chap.  v.). 


INTERNAL   ANATOMY, 


97 


The  ventral  nerve-cord  of  the  Annelids  is  no  doubt  in 
part  physiologically  equivalent  to  the  spinal  cord  of  the 
Vertebrates ;  but  since  the  two  structures  lie  on  opposed 
sides  of  the  body,  it  is  difficult  to  regard  them  as  morpho- 
logically equivalent.  Those  who  defend  the  Annelid-theory 
have  postulated  the  occurrence  of  a  half-revolution  of  the 
body  in  the  supposed  Annelid-like  ancestors  of  the  Verte- 
brates, as  a  result  of  which  they  acquired  the  habit  of  per- 
forming their  locomotion,  perhaps  swimming,  on  their  backs 
so  that  the  ventral  surface  was  turned  uppermost.  In  this 
way,  we  are  to  suppose  the  original  dorsal  and  ventral 
surfaces  became  reversed.  This  phylogenetic  acrobatic 
feat  with  all  its  consequences  is  hard  to  imagine,  and 
there  are  other  alternatives  which  make  it  an  unnecessary 
assumption.     (See  below,  V.) 

The  chief  fundamental  differences  between  the  dorsal 
spinal  cord  of  Amphioxus  and  of  Vertebrates  generally, 
and  the  ventral  ganglionic  chain  of  the  Annelids,  may  be 
summed  up  as  follows  :  — 


h 


Amphioxus. 

Annelids. 

Nerve-cord  is  hollow. 

Nerve-cord  is  solid. 

"  dorsal. 

"  ventral. 

"         "  unconstricted. 

"         "  constricted. 

"         "  single. 

"  double. 

Ganglion-cells    lie    inside   the 

Ganglion-cells    lie    outside   the 

fibrous  1.  yer. 

fibrous  layer. 

As  for  the  resemblances,  in  both  cases  nerves  are  given 
off  segmentally,  and  also  giant-fibres  are  present,  whose 
function,  however,  is  apparently  very  different  in  the  two 


cases. 


10 


w 


^ 


98 


ANA7'0MY  OF  AMPJIIOXUS. 


NOTES. 


I 


-   ■]',:■ 


\'M 


1.  (p.  49.)  Lankkstkr  has  made  the  suggestion  that  there- 
are  nut  distinct  capillaries  and  ccelomic  space  around  the  hepatic 
ctecum,  but  that  the  space  itself  is  capillariform.  'J'his  view  is  in 
accordance  with  v/hat  one  observes  in  transverse  sections. 

2.  (p.  50.)  The  fullest  account  of  the  contractile  blooil- 
vessels  of  Amphioxus,  as  observed  in  the  living  animal,  is  that 
given  by  Johannks  Mullf.r.  He  observed  the  peristaltic  con- 
tractions of  the  branchial  artery  (which  is  filled  with  a  perfectly 
colourless  blood),  beginning  from  its  hinder  end,  where  it  is  joined 
by  the  hepatic  vein  (which  also  undergoes  peristaltic  contraction 
from  before  backwards  along  dorsal  side  of  ccccum)  and  extend- 
ing to  the  front  end  of  the  pharynx.  The  intervals  between  the 
successive  contrictions  last  about  a  minute.  Immediately  suc- 
ceeding upon  the  contraction  of  the  branchial  artery,  the  bulbils, 
which  occur  at  the  base  of  the  primary  or  forked  gill-bars,  contract 
too.  He  says  that  the  heart-like  "aortic  arch"  which  occurs  to 
the  right  of  the  velum  (he  thought  there  was  one  on  the  left  side 
as  well)  contracts  from  below  upwards,  and  that  its  contraction 
enabled  him  to  discover  it.  As  mentioned  m  the  text,  van  Wijhe 
states  that  it  has  no  communication  with  the  branchial  artery. 
Johannes  Miiller  also  observed  the  peristaltic  contraction  of  the 
sub-intestinal  (portal  vein),  and  states  that  it  extends  to  the 
anterior  end  of  the  coecum.  It  should  be  remembered  that  his 
observations  were  made  on  young  transparent  individuals,  and  the 
statement  as  to  the  extent  of  the  contraction  of  the  sub-intestinal 
vein  is  open  to  doubt. 

3.  (p.  51.)  \  genital  a rft'iy  running  longitudinally  above  the 
gonadic  pouches  has  been  figured  by  Langerhans,  Rolph,  Schneider, 
Lankester,  and  Boveri,  but  its  relations  to  the  rest  of  the  vascular 
system  have  not  been  made  out.  It  is  doubtful  whether  its 
presence  is  constant. 

4.  (p.  58.)  The  "brown  funnels"  were  c"'scovered  by  Lan- 
kester in  1875,  and  were  subsequently  compared  by  Bateson  with 
the  collar-pores  of  Balanoglossus.  (See  Chap.  V.)  This  com- 
parison was  made  on  the  supposition  that  the  posterior  free  oper- 


NOTES. 


99 


cular  fold  of  the  so-called  collar  in  Balanoglossus  is  of  the  same 
nature  as  the  atrium  of  Amphioxus  ;  but  this  is  somewhat  tloubtful. 

5.  (p.  70.)  For  an  admirable  critical  and  historical  account 
of  our  knowledge  of  the  development  of  the  excretory  system  in 
the  different  groups  of  Vertebrates,  the  reader  may  be  referred  to 
the  report  on  the  "  Enhuickelinii^  der  Excrctionsorgane^'  by  Pro- 
fessor RucKKRT,  in  Merkel  antl  lionnet,  Ergebnisse  der  Anatomic 
iind  Entwickiiu)igsgcschichte,\\di\\^  I.,  1891.  It  will  be  sufficient 
to  note  here  that  the  ectodermic  origin  of  the  pronephric  duct, 
as  briefly  described  in  the  text,  only  holds  for  the  Selachians  and 
Mammals.  It  was  first  discovered  in  the  latter  by  Graf  Spek  in 
1S84,  and  confirmed  later  by  Fi.emmixg.  In  the  former  it  was 
discovered  independently  by  van  Wijhk  and  Ruckert  (1886-8). 
On  the  contrary,  in  Peiromyzon,  Amphibia,  Reptiles,  and  Birds,  the 
duct  does  not  arise  from  the  ectoderm. 

Van  W'ijhe  denied  the  segmental  fusions  with  the  ectoderm  of 
the  pronephric  tubules  in  Selachians  as  described  by  Ruckert. 
The  account  given  by  the  latter  author  has,  however,  been 
indirectly  confirmed  by  the  observations  of  Felix  on  the  chick, 
where  the  pronephric  outgrowths  were  found  in  some  cases  to 
undergo  a  transitory  fusion  with  the  ectoderm. 

BovERi  has  attempted  to  show  how  the  origin  of  the  pronephric 
duct  can  be  imagined  to  have  been  gradually  transferred  from  the 
ectoderm  to  the  mesoderm.  Finally,  it  may  be  noted  that, 
whereas  Ruckert  compared  the  pronephric  tubules  with  the 
Annelid  nephridia.  Semper  and  others  employed  the  mesonephric 
tubules  for  the  comparison.  The  fallacy  of  the  latter  comparison 
was  first  pointed  out  by  Furhrint.er. 

6.  (p.  74.)  In  1887  Paul  Mayer  discovered  that  the  sub- 
inle'^tinal  vein  in  the  Selachian  (Pristiurus)  embryo  communicated 
with  the  dorsal  aorta,  by  a  series  of  six  segmental  vessels  which 
passed  up  around  the  intestine  on  the  right  side  only.  Correspond- 
ing to  them  on  the  left  side  he  found  short,  blind  outgrowths  from 
the  dorsal  aorta  similar  to  those  figured  in  the  text  in  connexion 
with  the  pronephros  of  Ichthyophis.  Paul  Mayer's  connecting 
vessels  soon  become  aborted  with  the  exception  of  one  which 
enlarges  and  forms  the  proximal  portion  of  the  umbilical  artery. 
In  the    following   year   it  was  shown  in  a  brilliant   manner   by 


■^ 


!   t 


lOO 


ANATOMY   OF  AM  PI  11  OX  US. 


RiJcKERT  that  these  vessels  occur  in  the  same  segments  as  the 
rudimentary  pronephric  tubules,  and  give  rise  to  rudimentary 
glomeruh  at  the  level  of  the  tubules.  (Cf.  Fig.  35  B.)  There 
can  be  no  doubt  that  these  vessels  are  homologous  with  the 
vessels  which  run  through  the  primary  branchial  bars  of  Amiihi- 
oxus,  and,  as  shown  by  Bonkri,  assist  in  foyning  glomeruli  at  the 
level  of  the  excretory  tubules. 

The  morphological  importance  of  these  facts  is  very  great  and 
has  been  strongly  emphasised  by  Boveri.  Whether  Paul  Mayer's 
connecting  vessels  indicate  the  former  existence  of  gill-slits  in  that 
region  is  not  so  certain,  since  it  is  difficult  to  decide  whether 
the  indefinite  number  of  gill-slits  in  the  adult  Amphioxus  is  a 
palingenetic  (ancestral)  feature  or  not.  It  should  also  be  remem- 
bered that  Paul  Mayer  found  numbers  of  connecting  vessels, 
between  sub-intestinal  vein  and  dorsal  aorta,  in  the  tail. 

7.  (p.  78.)  Boveri  found  that  the  epithelium  of  the  pronephric 
duct  of  Myxine  was  of  a  glandular  nature,  comparable  in  this 
respect  to  the  atrial  epithelium  of  Amphioxus, 

8.  (p.  86.)  As  shown  in  Fig.  43,  R ohde  was  inclined  to 
follow  Schneider  in  the  belief  that  the  fibres  of  the  ventral  spinal 
nerves  were  directly  continuous  with  the  muscle-plates  and,  more- 
over, exhibited  the  same  striation  as  the  latter.  It  has  recently 
been  shown  by  Gustav  Retzius  that  this  appearance  of  continuity 
is  an  illusion,  as  in  so  many  other  cases  where  nerves  have  been 
wrongly  supposed  to  enter  into  direct  continuity  with  peripheral 
end-organs.  By  employing  Ehrlich's  method  of  staining  nervous 
tissue,  intra  vitam,  with  methylene  blue,  Retzius  has  proved  that 
the  motor  fibres  of  Amphioxus  pass  with  a  somewhat  winding  course 
between  the  muscle- plates,  and  simply  end  on  the  surface  of  the 
plates.  Rarely  they  branch  dichotomously,  but  there  is  no  special 
end-apparatus  as  in  the  higher  forms.  Their  connexion  with  the 
muscle-plates  is,  therefore,  one  of  intimate  contiguity,  but  not  of 
continuity. 

9.  (p.  91.)  The  cerebral  vesicle  of  Amphioxus  was  discovered 
in  1 85  8  by  Leuckart  and  Pagenstecher.  Owsjaxnikhw  (186S) 
thought  it  represented  the  fourth  ventricle  of  the  vertebrate  brain. 
Stieda  (1873)  was  the  first  to  homologise  the  cerebral  vesicle  of 
Amphioxus  with  the  entire  brain  of  the  higher  forms,  and  to  regard 


NOTES. 


lOI 


it  as  rejiresenting  the  latter  in  its  simplest  form  without  any  trace 
of  subdivision.  This  view  has  very  generally  been  adopted.  Stieila 
also  recognised  the  dorsal  and  ventral  groups  of  ganglion-cells  (of 
which  the  former  is  shown  in  section  in  Fig.  46)  as  belonging  to 
the  hinder  portion  of  the  brain.  Rohde'i  conception  of  the  brain 
of  Amphioxus  agreed  very  closely  with  that  of  Stieda,  but  he  made 
a  more  detailed  study  of  its  histological  character,  and  defined  its 
limits  more  precisely.  He  concludes  that  the  beginning  of  the 
spinal  cord  proper,  in    ;.e  absence  of  any  outward  mark  of  dis- 


h 

«* 


i 


Fig.  51.  —  Sagittal  section  tlirough  tin;  cerebral  vehicle  of  Amphioxus.  (After 
Kri'i-i-EK.) 

(.T'.  Cavity  of  cerebral  vesicle,  e.  Kye-spot.  s^.c.  Dorsal  group  of  ganglion- 
ceils  (cf.  F"ig.  46).  inf.  Infundibular  depression,  l.o.  Lobus  olfactorius  inijiar. 
tp.  Tuberculum  posterius. 

tinction  from  the  brain-region,  would  lie  at  the  point  marked  by 
the  appearance  of  the  first  of  the  giant  ganglion-cells,  which  he 
denotes  by  the  letter  A.     (Cf.  Fig.  4S.) 

Quite  recently  the  attempt  has  been  made  by  Professor  V(^\ 
KuPFFER  to  determine  in  detail  the  delimitation  of  the  cerebral 
vesicle  of  Amphioxus  (Fig.  51).  The  slight  outpushing  of  the 
wall  of  the  vesicle  towards  the  base  of  the  olfactory  pit  has  been 
mentioned  in   the  text.      It  was  discovered  by  Lanckrhans  in 


102 


AA'A  TOMV  Of  A  MP/I/JXi'S. 


1876,  who  called  it  the  /o/>iis  olfactoriiis.  Kupffer  ha^  succeeded 
in  finding  a  similar  siriicturc  in  the  embryos  of  other  Vertebrates, 
notably  in  Acipenscr  stitrio  (the  sturgeon).  He  calls  it  the  /<'/'//» 
olfactorius  impar,  and  shows  that  it  indicates  the  ])oint  where  the 
medullary  tube  remained  for  the  longest  and  last  time  in  direct 
connexion  with  the  external  ectoderm,  precisely  as  is  the  case  in 
Amjihioxus.  There  is  thus  at  least  one  fixed  point  common  td 
the  cerebral  vesicle  of  Amphioxus  and  the  brain  of  the  craniate 
Vertebrates.  Hut  Kupffer  has  found  another.  While  it  is  obvious 
that  the  anterior  wall  of  the  vesicle  containing  the  pigment  whicii 
constitutes  the  eye-spot  is  homologous  with  the  primary  optic  tract 
{feccssiis  opticus)  of  the  higher  Vertebrates,  in  which  pigment  is. 
in  many  cases,  deposited  in  the  embryo,  Kupffer  states  that  he 
is  able  to  detect  an  infundibular  depression  in  the  floor  of  the 
cerebral  vesicle  of  Amphioxus.  Immediately  behind  this  depres- 
sion there  is  a  prominence  in  the  wall  of  the  vesicle,  which  Kupffer 
calls  the  tuberculinn  postcrius.  This  point  is  also  to  be  identified 
in  the  brains  of  the  higher  Vertebrates. 

The  dorsal  dilatation  of  the  central  canal,  which  Hatschek  dis- 
covered and  compared  with  the  fourth  ventricle  of  the  vertebrate 
brain,  whose  roof  is  similarly  membranous  and  not  nervous  (Fig. 
45),  is  certainly  a  very  curious,  and  apparently  constant,  feature 
in  young  individuals,  as  I  can  affirm  in  confirmation  of  Hatschek. 
The  conclusion  come  to  by  Hatschek,  however,  that  the  loLus 
olfactorius  of  Langerhans  is  the  homologue  of  the  infundibulum  of 
the  higher  forms,  would  appear  to  be  untenable  in  the  light  of 
Kupffer's  researches. 

It  is  beyond  the  scope  of  this  book  to  discuss  the  difficult 
problem  of  the  origin  of  the  paired  eyes  of  the  Vertebrates,  but  it 
may  be  pointed  out  that  there  is  no  difficulty  in  identifying  a 
stage  in  the  embryonic  development  of  the  optic  tract  in  the 
Craniota  corresponding  to  the  permanent  condition  of  things 
in  Amphioxus.  This  fact  was  first  demonstrated  by  Wilhei.m 
MiJLLER  in  1874.  On  account  of  its  position  in  front  of  and 
below  the  cerebral  vesicle,  it  is  doubtful  whether  the  eye-spot  of 
Amphioxus  is  homologous  with  the  eye  of  the  Ascidian  tadpole. 
(See  below.) 

10.   (p.  94.)    It  is  a  significant  fact  that  giant  nerve-fibres  appear 


ill ' 


XO  TES. 


103 


to  be  present  in  the  spinal  cord  of  all  those  Vertebrates  whose  tail 
serves  as  an  im[)ortant  organ  of  loconu)ti()n.  'I'hus,  they  occur  in 
fishes,  tailed  Am|)hil)ia,  in  the  tadpoles  of  tailless  Amphibia,  and, 
finally,  they  have  been  recently  discovered  by  Max  Koppkn  in  tiie 
caiulal  region  of  the  si)iml  cord  of  the  li/ard.  In  tlie  frog  and  higiier 
forms  they  do  not  occur.  From  these  considerations  Kiippen 
thinks  that  there  is  a  causal  relationship  between  the  occurrence 
of  gi;mt-fibres  in  the  spinal  cord  and  the  presence  of  a  locomotor 
tail.  'I'he  caudal  locomotion,  characterised  by  the  rapid  swaying 
motion  of  the  tai',  is  not  confined  to  the  post-anal  region  in 
.\mphioxus,  but  involves  the  whole  body. 

Contrary  to  the  observations  of  Kisk;,  both  Nansfn  and  Rohdk 
are  of  opinion  that  the  giant-fibres  of  Annelids  (Polycha^ta)  have 
the  same  physiological  significance  for  the  central  nervous  system 
as  those  of  Amphioxus. 

Some  of  the  older  authors  mistook  the  giant  nerve-fibres  for 
capillary  blood-vessels.  .\s  a  matter  of  fact  no  blood-vessels 
traverse  the  central  nervous  system  of  Amphioxus.  It  may  be 
added,  also,  that  there  are  no  mtuiullatcil  nerve-fibres. 

II.  (p.  95.)  Several  suggestions  have  been  made  as  to  pos- 
sible representatives  of  the  spinal  ganglia  of  the  dorsal  roots  of 
the  Craniota  in  .\mphioxus. 

Omitting  earlier,  and  obviously  erroneous,  suggestions,  Rohde 
(1S88)  regarded  the  nuclei,  which  he  found  imbedded  in  the 
dorsal  roots,  as  a  collection  of  *'  nervous  nuclei,"  comparable  to 
the  spinal  ganglia  of  the  higher  Vertebrates  (Fig.  46).  .According 
to  Rktzius  (i8go)  these  nuclei  are  not  of  a  nervous  r.ature  (prob- 
ably belong  to  supporting-cells),  ana  he  tentatively  suggests  that 
the  spinal  ganglia  are  represented  by  groups  of  bipolar  ganglion- 
cells  which  occur  inside  the  spinal  cord  at  fairly  regular  intervals 
in  two  longitudinal  rows,  one  on  each  side  of  middle  line.  The 
main  process  (axis-cylinder)  of  these  cells  divides  in  T-form,  and 
one  of  the  branches  of  the  T  passes  into  the  dorsal  root.  (Cf. 
Fig.  50.) 

Finally,  Hatschek  (189?)  finds  the  homologues  of  the  spinal 
ganglia  at  the  points  where  the  dorsal  nerves  divide  into  ramus 
dorsalis  and  ramus  vcntralis. 


Wm 


^pfl; 


III 


DKVKlA)rMi:XT   OF    AMI'IIIOXUS. 


As  an  introduction  to  the  study  of  embryolo<;y,  and  as 
an  indispensable  aid  to  a  reasonable  appreciation  of  the 
value  of  enibryc)loi,ncal  facts,  the  life-history  of  Amphioxus 
provides  an  object  which,  for  its  capability  of  application 
to  almost  every  branch  of  zoi)lo<;ical  discussion,  is  perhaps 
unrivalled.  It  is  alike  useful  in  a  text-book  of  human  em- 
bryology, and  in  one  of  invertebrate  zoology. 

The  reason  for  this  obviously  lies  in  the  fact  that  in 
Amphioxus  everything  has  its  own  definite  line  of  de- 
marcation, all  the  fundamental  structures  of  the  body  are 
laid  down  with  schematic  clearness,  there  are  no  massive 
aiiglomerations  of  cells  forming  complicated  tissues,  but  all 
the  organs  are  of  simple  epithelial  origin  and  constitution. 

Whereas  in  many  of  the  higher  and  lower  animals  the 
greatest  difficulty  is  often  experienced  in  deciding  to  which 
of  the  primary  layers  of  the  body  this  or  that  structure 
owes  its  origin,  in  Amphioxus  there  is  no  such  difficulty. 
With  these  advantages  it  is,  therefore,  no  wonder  that 
Amphioxus  should  serve  as  a  refuge  to  the  perplexed 
embryologist. 

It  is  not  an  exaggeration  to  say  that  the  researches  both 
of  KowAi.KVSKV  and  of  Hatschek,  on  the  development  of 
Amphioxus,  will  always  rank  among  the  classics  of  embry- 
ological  literature  ;  while  it  is  a  familiar  fact  that  Kowa- 
levsky's  earlier  work   (1867)  on    the   development  of  the 

104 


m. 


/;.1//M'  yOA'/C  DE  VEI. 0PM EXT. 


105 


Ascidians   and  of  Amphioxus    marks  a  distinct  epoch   in 
the  progress  of  the  science  of  emhryology. 


! 


EMIiRVOMC   DKVKI.OI'MENT. 
Fertilisation  a)ui  Sci^mcntation  of  the  Ovitm. 

The  breeding-season  of  Amphioxus  extends,  in  the  Med- 
iterranean, from  spring  to  autumn. 

The  gonailic  pouches  become  very  much  distended  by 
the  ripening  of  the  ova  and  spermatozoa  in  the  respective 
sexes,  and  hnally  burst,  cHscharging  their  contents  into  the 
atrial  cavity,  whence  they 
reach  the  exterior  through 
the  atriopore.'  At  the  time 

of   complete  sexual  matu-       J^^l^^^^        '..^^^"il^V-^ 
rity  the  gonads  become  so 

large    that    the   atrium    is     »KSi*w«owfi3Km^is».5^^  >. 

used  up  to  its  utmost 
capacity,  and  its  walls  be- 
come stretched  to  such  an 
extent      that     the     meta- 

pleural  folds  are  flattened       Fig.  52.- Unfe.t.ii..(i..vum.,f  Amphi- 
oxus.     Maynififd    about    750   diameters. 

up  against  the  sides  of  the  (.\fter  lwckkuans.) 

,       1  _  J.  \()llk   granules,    f.  Follicle.     ;/.   Nu- 

V"  cleus    (f^erminai    vesicle),   with   nuiicolus. 

The       ovum        is        semi-   /•  I'rotuplasmic  area,  free  from  yolk  gran- 
ules, surrounding  the  nucleus. 

opaque,  contains  granules 

of  yolk  equally  distributed  throughout  its  substance,  and 
is  surrounded  by  a  cellular  membrane  known  as  \.\iQ.  follicle 
of  the  Q'^g,  and  sometimes  less  accurately  spoken  of  as 
the  vitelline  vieinbrane  (Fig.  52). 

Spawning,  when  it  occurs,  invariably  takes  place  at  sun- 
down,—  i.e.  between  five  and  seven  o'clock  in  the  evening, 
—  and  never,  so  far  as  is  known,  at  any  other  time. 


^Ifl 


?:! 


}iih 


m 


106 


DEVELOPMENT   OF  AM  PHI  OKU S. 


Ova  and  spermatozoa  arc  discharged  simultaneously  by 
male  and  female  individuals  into  the  water,  and  fertilisa- 
tion is  effected  in  the  latter  medium. 

The  final  stages  in  the  maturation  of  the  ovum  of  Am- 
phioxus  are  very  imperfectly  known,  and  the  extrusion  of 
the  so-called /(V*^?;-  bodies,  preparatory  to  the  process  of  fer- 
tilisation, has  not    been  properly  studied,  only  one   such 


1 1 


Fig.  53.  —  I'eriilist'd   ovum    of    Atnptiioxus.      Highly   magnified.      (From   a 
drawing  Uitiiilv  lent  by  I'rofossor  li.  H.  Wil.soN.) 

i/.i.  1  Mrcctivc  corpuscle  or  polar  body.     c'.  C'vum.     A  l-'ollicle. 


body  having  been  observed,  whereas  from  the  analogy  of 
all  other  sexually  reproducing  animals  we  should  expect 
two  polar  bodies  (directive  corpuscles)  to  be  given  off  be- 
fore the  egg  was  fully  mature.  As  soon  as  an  ovum  has 
been  fecundated  by  the  entrance  of  a  spermatozoon,  the 
vitelline  membrane  springs  away  from  the  body  of  the  egg- 
cell,  leaving  a  wide  space  between  the  latter  and  the 
former  (Fig.  53).     This  expansion  of   the  vitelline  mem- 


£MBR  \  -( IV/C  DE  J  'EL  0PM EXT. 


107 


brane  is  the  first  outward  and  visible  sign  of  the  accom- 
plishment ot  the  process  of  fertilisation. 

About  an  hour  later,  — that  is  to  say,  at  about  8  p.m., — 
the  Qg^  becomes  flattened  at  its  two  poles,  and  a  depression 


Fig.  54.  —  Division  of  ovum  into  the  first  two  binstomercs.    The  polar  body 
niarl;s  the  animal  pole.     (.Alter  Haisciii-.K.) 

appears  at  the  animal  pole,  the  latter  being  indicated  by 
the  polar  body.  The  depression  deepens  and  extends  as  a 
meridional  furrow  round  the  egg.  Finally,  the  division  of 
the  egg  into  two  hah'es  or  blastomcres,  which  remain  at- 
tached to  one  another,  is  completed,  and  the  first  stage  in 
the  segmentation  of  the  Qg^  is  accomplished  (Fig.  54). 

As  it  is  beyond  i.he  scoi)e  of 
this  book  to  discuss  the  mechan- 
ics of  cell-division,  the  descrip- 
tion of  the  segmentation  stages 
will  be  very  brief. 

The  first  meridional  cleavage 
which  divides  the  egg  into  two 
blastomcrcs  is    followed    by    an- 
other one   It   riu-ht   Tn<drs  to   it  ^'2-  55--I'''glit-ceIl  stage  seen 
Orner    one    at    llgnt    angles  ro    11,     fromtheupper  (animal)  pole.    F,mr 

dividing  each  of    the  two    blastO-    »>"•'"  l)la.sl.. meres  (micn)meres)  lie 

uptm   four  larger  hlastcjmeres  (tna- 
mcrCS    agaUl    nitO    two.        In    this     cromeres).    Radial  type  of  cleavage. 

way  the   stage  with  four  equal   (After  !•:.  m.  \vii.s..n.) 
blastomeres  in  one  plane  is  produced.     Next   follows  an 
equatorial  cleavage,   by  which  eight  blastomeres  are  pro- 
duced, the  four  upper  cells  at  the  animal  pole  being  some- 


p;i  :.?:"■   g 


f:^ 


lY 


I  ill):) 


i  h\ 

ill 


■'..h    ':-i 


I  08 


£>£  I  '/■:/.  OPMEXT   OF  AM  PI  Hi  >.\'(  '.V. 


il; 


what  smaller  than  the  four  lower  cells  at  the  vegetative 

pole,  since  the  latter  contain   a   greater  quantity  of  the 

yolk-spherules  (Fig.   55). 

The  next  cleavage  giving  rise  to  an  embryo  of  sixteen 

cells  is  meridional.     Then  the  eight  ui^^jci  and  the  eight 

lower  cells  become  respectively 
divided  by  equatorial  cleavages, 
and  so  the  thirty-ttv'o  cell  stage 
is  reached  (Fig.  56). 

The  embryo  is  now  known 
as  a  blastula,  and  consists  of  a 
mulborrv-like  mass  of  cells  sur- 
rounding  a  central  cavity  called 

Fig.  56.  —  Tliirtv-two  cell  stage,  ^  .  .  ,, 

consisting  of  four  rows  of  eight  cells,  thC    SCgmCUtatWU-CaVlty    Or    OUlS- 

each  ranged   around  a  central  seg-  f,-,r,yf 
mentation  cavity  (blastocuel).    The 

polar  body  is  still  visible  at  the  ani-  From     this    point     of     the     de- 

nial tiole.     (.After  Hatschkk.)  ,  i         ,, 

vclopment  the  blastomeres  go 
on  dividing  with  more  or  less  regularity,  until  the  wall  of 
the  blastula  consists  of  a  great  number  of  cells  arranged 
in  a  single  layer  about  the  central  cavity. 

The  segmentation  of  the  egg  of  Amphioxus,  however, 
by  no  means  follows  the  uniform  and  stereotyped  plan 
that  has  been  hitherto  supposed.  It  has  recently  been 
discovered  by  Professor  E,  B.  Wilson  that  Amphioxus 
presents  an  example  of  a  polymorphic  cleavage.  Instead 
of  following  one  type,  it  follows  three  types  of  cleavage  ; 
namely,  a  mdial  type  (I'^igs.  55  and  56),  a  bilateral  type 
(Fig.  57),  and  a  spiral  type  (Fig.  58).  These  three  types 
of  cleavage  are  reducible  to  a  common  basis,  and  are  con- 
nected together  by  all  possible  intermediate  gradations. 
Wilson  points  out  that  in  the  bilateral  type  of  cleavage 
Amphioxus  shows  a  close  correspondence  with  the  Ascid- 
ian  embryo. 


iili 


KMBR  1  \h\IC  JJE I •ELOJ'MEXT, 


109 


The  segmentation  or  cleavage  of  the  ovum  results  in 
the  formation  of  a  spherical  blastula,  closed  at  all  points, 


Fig.  57.  —  Three  stages  in  the  segmentation  of  the  ovum,  according  to  the 
bilateral  tyjie.     F>om  the  lower  [lole.     (After  K.  B.  WiLSON.) 

,/.  iMgiit-cell  stage.  .-/,  />',  C,  I).  The  four  macromeres,  above  wliich  are  set  n 
portions  of  the  four  microrneres. 

/-/.  Plane  of  first  cleavage,  with  resjK'ct  to  which  tiie  cells  divide  in  such  a  way 
as  to  become  arranged  in  a  bilaterally  symmetrical  manner. 

//-//.  Plane  of  second  cleavage. 

/)'.  Transition  to  the  sixteen-cell  stage. 

C.  Sixteen-cell  stage.  The  line  in  each  cell  indicates  the  direction  in  which  the 
next  division  of  the  cell  would  take  place. 

and  consisting  of  some  256  cells  surrounding  a  spacious 
cavity,  the  blastoccel. 

The  stages  of  development  lead- 
ing up  to  the  blastula  are  known 
as  the  segmentation  stages.  At 
their  completion,  although,  of 
course,  cell-division  continues  to 
take  place  actively,  yet  other  pro- 
cesses supervene  which  render  the       Fig.  58.  —  Kight-ceii  stage 

....  _     ,       .      ,.    .  ,       ,        „       from    the  u]iper  jiole,  illlustrat- 

mere  division  of  the  individual  cells    ing  the  spiral  typo  of  cleavage. 

of  minor  morphological  importance.    ^^^*'^''  ^"  '^"  ^^^''-^o^"-) 


Gastnilation. 

The  next  phase  of  the  development  is  known  as  the 
gastrulation  of  the  embryo.  The  cells  forming  the  lower 
or  vegetative  side  of  the  blastula  remain,  throughout  the 
segmentation  stages,  somewhat  larger  than  the  rest  of  the 


'^ 


•t* 


no 


/;/; ; 'a/, opmext  of  .iMriiioxi s. 


blastula-wall.  This  side  now  becomes  flattened,  as  shown 
in  Fig.  59  A.  Next,  the  flattened  side  of  the  blastula 
becomes    gradually    tucked    up    or   invaginated    into    the 

blastocoL'l  (Fig.  59  />)  until, 
finally,  the  segmentation 
cavity  is  completely  obliter- 
ated, and  the  invaginated 
layer  of  cells  becomes  tightly 
fitted  up  against  the  outer 
layer  (Fig.  59  C). 

The  embryo,  now  known 
as  the  gastrula,  is  a  double- 
layered  sac,  the  cavity  of 
which  was  produced  by  in- 
vagination, and  is  known  as 
\}[iQ.  primitive  gastral  cavity, 
or  arcJicntcwn.  This  cavity 
is  widely  open  to  the  ex- 
terior by  the  orifice  of  invagi- 
nati(^n,  or  blastopore,  which 
in  German  is  designated  by 
the  expressive  term  Unnnnd. 
The  two  layers  of  cells  which 
constitute  the  wall  of  the 
gastrula  are  the  primitive 
gcrm-laycrs ;  the  outer  layer 
is  the  primitive  ectoderm, 
and  the  inner  layer,  sur- 
rounding the  gastral  cavity, 
is  the  primitive  endoderm  ;  the  two  layers  are  continuous 
with  one  another  round  the  margin  of  the  blastopore. 

The   blastopore  is  at  first    a   very   wide  oval  opening, 
but  it  soon  becomes  narrowed  down  to  a  small  aperture 


£  "^  =^"  ^" 


IP 


EM  BR  1  OXIC  DE I  'EL  ( V  \Ml:XT. 


Ill 


by  the  continued  deepening  of  the  archenteric  cavity 
(Fig.  60). 

It  is  now  a  well-estabhshed  fact  that  all  multicellular 
animals  (Metazoa)  pass  through  a  gastrula-stage  in  the 
course  of  their  development,  although  the  form  of  the 
gastrula  is  often  extremely  modified  and  difficult  to  recog- 
nise. The  significance  of  this 
fact,  as  was  long  since  pointed 
out  bv  Huxlev,  Haeckel,  Lan- 
kester,  and  others,  is  very 
great  when  it  is  remembered 
that  the  embryonic  character- 
istics of  the  gastrula  are 
essentially  identical  with  the 
permanent  features  of  the 
organisation  of  the  Coelen- 
terata  (Hydra,  etc.). 

Returning  to  the  gastrula  begins  to  rotate  witiun  tiic  foiiicic. 

,,       ,  ,    .      ^  .  ,  (.After  IlATSCIlKK.) 

ot  Amphioxus,  \\\  the  course 

of  the  further  differentiation  which  goes  hand  in  hand 
with  the  progressive  growth  antl  development,  we  shall 
find  that  the  primitive  ectoderm  gives  rise  to  (i)  the 
central  nervous  system  and  (3)  the  definitive  ectoderm  ;  the 
primitive  endoderm  gives  rise  to  (i)  the  viesoderni,  \\\\\q\\ 
is  usually  regarded  as  a  third  or  intermediate  germ-layer  ; 
(2)  the  notochord ;  and  (3)  the  definitive  endoderm,  which 
forms  the  lining  mucous  epithelium  of  the  alimentary 
canal  ;  finally,  the  primitive  gastral  cavity  or  archenteron 
will  become  subdivided  into  ( 1 )  the  body-cavity  or  civ/oin, 
and  (2 )  the  definitive  gut  or  alimentary  canal. 

The  embryo  shown  in  optical  section  in  Fig.  60  repre- 
sents the  stage  reached  at  midnight  of  the  first  night  of 
development.     It  will  be  noticed  that  one  side  is  convex, 


Fig.  60.  —  Optical  lonfjiiiKlinal  sec- 
tion of  kiter  gustruki.  Cili;i  (Hms^uILi) 
have  been  iMotmied  from  the  ectocierni 
cells,   and    the    enil)ryo    at   tliis    stage 


A 


112 


Dl:  1 1:1.  OPME.WT   Of    AMI  '///OATS. 


A 


ij„ii 


m 


"Si 


while  the  opposite  side  is  flattened  ;  this  is  an  indication 
that  dorso-ventral  differentiation  has  taken  place,  since 
the  flattened  side  marks  the  dorsum  or  back  of  the  embryo, 
while  the  convex  side  is  ventral.  It  may  be  seen  further 
that  the  blastopore  is  inclined  towards  the  dorsal  side  of 
the  embryo.  The  dorsal  inclination  of  the  blastopore  is 
eminently  characteristic  of  the  vertebrate  yastrula  from 
the  Ascidians  up  to  the  highest 
craniate  forms.  In  the  Inverte- 
brates (Annelids,  IMolluscs,  etc.) 
the  blastopore  acquires  a  ventral 
inclination.* 

At  the  stage  represented  in  Fig. 
60  the  embryo  commences  to  ro- 
tate within  the  vitelline  membrane, 
each  ectodermic  cell  being  now 
provided  with  a  vibratile  cilium. 

The  embryo  next  b('gins  to  elon- 
gate, and  the  blastopore  becomes  ^'°'"   *''*^   ectoderm.      (After 
still  narrower  (Fig.  61). 

A  comparison  of  the  accompanying  figures  will  show 
that  the  narrowing  of  the  blastopore  is  effected  by  the 
downward  and  backward  growth  of  its  dorsal  border, 
while  the  ventral  lip  remains  stationary.  The  dorsal  ecto- 
derm, which  is  converted  into  the  Dicdiillary  plate,  now 
showT  indications  of  a  shallow  longitudinal  groove.  This 
is  the  beginning  of  the  medullary  groove  which  leads  on 
to  the  formation  of  the  central  nervous  system. 

*  For  a  discussion  of  the  phylogenetic  relation  of  the  blastopore  or  protu- 
stonia  (Hatschek)  to  the  mouth  and  anus,  the  following  works  should  be 
consultetl :  ADAM  SiiDGWiCK,  On  the  Orii^nii  of  Melumeric  Segnieutation,  etc.. 
Quarterly  Jour.  Micro.  Sc,  XX I\'.,  1S84,  and  by  the  same  author,  A'otcs  on 
Elasmohranch  /development,  /b.  Vol.  XX>'ni.,  1S91-92. 

I'inally,  Bertholu  Hatschkk,  /.ehrbiuh  der  Zoologie,  Jena,  1888-91. 


Fig.  61.  —  Elongated  gas- 
trula.  Optical  longitudinal  sec- 
tion.     The    cilia    are    omitted 


EMHk  \  \  )XIC  DE I  'EL  OI'MEXT, 


113 


Growt/i  of  Frce-sivunmins;  Embryo. 

Between  4  and  5  a.m.  in  the  first  morning  of  develop- 
ment, i.e.  at  about  the  eighth  hour,  the  embryo  has  reached 
the  stage  represented  in  Fig.  62,  and  it  now  bursts  through 
the  vitelline  membrane  and  becomes  free,  swimminir  bv 
means  of  its  cilia  at  the  surface  of  the  sea,  or  aquarium. 

The  fact  that  Amphioxus  has  a  free-swimming,  ciliated 
embryo  is  important  as  providing  a  general  connecting 
link  between  the  Vertebrates  and  the  Invertebrates,  since 


it 


eiTC 


..^ntft 


Fig.  62.  —  Embryo  of  Amphioxus  at  the  stage  at  which  it  ruptures  the  follicle 
and  becomes  free-swimming. 

./.   Seen  from  above  as  a  semi-opaque  object.     (After  Kowalevsky.) 

li.  Seen  in  sagittal  (optical)  section.     (After  Hatschek.) 

arc.  Archenferon.  w./.  Medullary  plate,  my.c.  Myocoelomic  pouches  of 
archenteron.    p.n.c.  Posterior  neurenteric  canal. 

the  possession  of  a  ciliated  ectoderm  is  very  common 
among  Invertebrate  embryos,  but  entirely  unknown  among 
the  craniate  Vertebrates. 

The  medullary  plate  is  now  being  closed  off  from  the 
niiter  surface.  This  is  effected  by  the  co-operation  of  two 
factors.  The  ectoderm  which  bounds  the  medullary  plate 
laterally,  grows  over  it,  and  simultaneously  the  ectoderm  of 
the  posterior  or  ventral  lip  of  the  blastopore  grows  for- 
ward over  the  medullary  plate  so  as  to  shut  in  the  blasto- 
pore from  the  exterior  (Fig.  62  A  and  />').     The  archenteric 


I     3 


<^ 


f 


114 


DEVIJA)PMEXT   OF  AMPUIOXIS. 


cavity  therefore  no  longer  opens  by  the  blastopore  to  the 
exterior,  but  it  communicates  with  tiie  medullary  tube. 
Tiie  blastopore  has,  in  fact,  become  converted  into  the 
ncHirnteyic  canal,  joining  the  canal  of  the  central  nervous 
system  with  the  cavity  of  the  alimentary  system.  This 
remarkable  condition  of  things  was  first  discovered  bv 
KoWALi-vsKV,  who  also  found  it  in  the  Ascidians  and  in 
a  number  of  the  higher  Vertebrates.  It  has  since  been 
found  to  occur  in  all  classes  of  Vertebrates,  including 
man. 

Hitherto  the  body- wall  of  the  embryo  has  consisted  of 
only  two  primary  germ-layers,  ectoderm  and  endoderni. 
At  the  stage  now  under  consideration,  a  third  interme- 
diate layer,  the  mesodcnn,  has  begun  to  put  in  its  appear- 
ance. The  mesoderm  arises  in  the  first  instance  as  a 
series  of  paired  lateral  pouches  of  the  archenteron.  In 
Fig.  62  the  first  two  or  three  archenteric  pouches  are 
distinctly  visible.  Before  proceeding,  however,  to  a  more 
detailed  account  of  the  origin  of  the  nervous  system  and 
of  the  mesoderm,  we  will  trace  briefly  the  changes  in 
external  appearance  which  the  embryos  undergo  up  to  tiie 
time  of  the  formation  of  the  mouth. 

As  the  embryos  are  very  transparent,  the  external 
appearance  necessarily  involves  a  good  deal  of  the  inter- 
nal structure. 

The  period  of  enibyyonic  development  may  be  defined  as 
commencing  with  the  first  cleavage  of  the  ovum,  and  end- 
ing with  the  perforation  of  the  mouth,  thus  comprising 
approximately  the  first  thirty-six  hours.  During  this 
period  the  embryo  does  not  take  up  independent  nourish- 
ment, but  lives  on  the  original  food-yolk  which  was  con- 
tained in  the  Q%^. 

During  the  first  few  hours  of  its  pelagic  or  free-swim- 


fc 


lii 


KM  BR  YOXIC  DE  \  'EL  0  PM  EXT. 


I»5 


ming  existence,  the  embryo  keeps  rigidly  to  the  surface 
of  the  water. 

After  its  escape  from  the  vitelline  membrane,  it  grows 
rapidly  in  length.  Fresh  archenteric  pouches  are  added 
to  those  already  formed,  one  after  the  other,  in  metameric 
order.  The  medullary  plate  {i.e.  the  fore-cast  of  the  nerve- 
tube)  becomes  completely  closed  in  beneath  the  superficial 
ectoderm  except  at  its  anterior  extremity,  where  it  remains 
open  to  the  exterior  in  the  mid-dorsal  line  by  an  aperture 
known  as  the  nciiroporc  (Fig.  6^^  A,  B,  C).  Finally,  the 
notochord  becomes  differentiated  from  the  primitive  endo- 
(Icrm. 

According  to  Hatschek  the  number  of  mcsodcrmic 
somites  which  arise  as  diverticula  from  the  archenteron 
is  fourteen  pairs.  Those  which  are  subsequently  added 
to  these  arise  at  the  hinder  end  of  the  body  by  prolifera- 
tion from  the  cells  which  lie  behind,  and  at  the  sides  of 
the  neurenteric  canal,  or  in  that  region,  so  that  they  never 
appear  as  actual  outgrowths  from  the  archenteron. ^ 

In  Fig.  6^  C  the  embryo  has  undergone  some  radical 
changes  in  form.  Its  body,  previously  cylindrical,  has 
become  laterally  compressed,  the  ectoderm  cells  of  the 
hinder  end  of  the  body  have  begun  to  elongate  so  as  to 
form  the  rudiment  of  a  provisional  caudal  fin,  and  the 
front  end  of  the  body  has  grown  out  into  the  shape  of 
a  snout.  In  connexion  with  the  latter  there  are  two 
remarkable  structures  which  arise  as  a  pair  of  outgrowths 
from  the  anterior  region  of  the  archenteron,  and  were  first 
described  by  Hatschek  as  a  pair  of  anterior  intestinal 
diverticula.     These  we  shall  return  to  later. 

Near  the  front  end  of  the  alimentary  canal  a  curious 
sac-like  structure  has  appeared  (Fig.  63  C).  It  arose  as 
a  transverse  groove  in  the  floor  of  the  gut  in  the  regi.m 


t   ■ 

r 

••. 


ii6 


DEVEI.OIWrr.XT   OF  AMPlIIOXi'S. 


of  the  first  myotome,  extending  from  the  right  side  under- 
neath to  the  left  side  of  the  body.  (Cf.  Figs.  6^  A  and  71.) 
The  groove  deepened,  and  its  margins  coalesced,  and  so  it 


rj.>:- 


Fig.  63.  —  Growth  of  the  ciliated  embryo  of 
Amphioxus.     (After  Hatschek,  slightly  altered.) 

A.  Stage,  with  nine  pairs  of  myocoelomic  pouches ; 
froir,  left  side. 

li.  Same  stage  from  dorsal  side. 

C.  Stage,  with  fifteen  pairs  of  myotomes;  from 
the  right  side.  Vacuoles  have  appeared  in  cells  of 
notochoru. 

ch.  Notochord.  c.s.g.  Club-shaped  gland,  f^.s. 
Rudiment  of  first  gill-slit.  int.  Intestine,  l.a.d.  Left 
head-cavity  (left  anterior  intestinal  diverticulum). 
my.c.  Myococlomic  (archenteric)  pouches,  tip.  Xeu- 
ropore.  n.t.  Medullary  tube.  pi;.  Pigment  granules 
in  tloor  of  medullary  tube,  p.ti.c.  Posterior  neuren- 
teric  canal,  r.a.d.  Right  head-cavity  (right  anterior 
intestinal  diverticulum). 


••rati 


la-ii- 


-tnt 


-~pn.c 


"i :/ 


became  constricted  from  the  gut,  and  now  forms  a  hollow 
sac  closed  at  present  at  both  ends.  It  is  known  as  the  club- 
shaped  gland.     Immediately  behind  it,  in  Fig.  6^  C,  is  seen 


il 

if 


i.jjiKvoxic  Ju:r/:/.oj'.]//-:.v7: 

a  shallow  depression  in  the  floor  of  the 
gut.  This  is  the  indication  of  the  first 
gill-slit  which  becomes  perforated  at 
this  point  later. 

From  this  stage  it  is  an  easy  tran- 
sition to  the  stage  which  marks  the 
close  of  the  embryonic  and  the  com- 
mencement of  the  /ana/  period  of 
development. 

In  the  embryo  shown  in  Fig.  64,  the 
mouth  appears  as  an  oval  aperture  placed 
asymmetrically  on  the  left  side.  At  its 
first  origin  it  is  relatively  much  smaller 
than  shown  in  the  figure.  A  disc-shaped 
thickening  of  the  ectoderm  appears  on 
the  left  side,  in  the  region  of  the  first 
myotome.  The  subjacent  endoderm 
fuses  with  the  thickening,  and  then  the 
centre  of  the  disc  becomes  perforated, 
and  so  the  mouth  is  formed. 

The  club-shaped  gland  has  acquired 
an  opening  to  the  exterior  immediately 
below  the  mouth,  on  the  left  side ; 
while  the  body  of  the  gland  lies  on  the 
right  side. 

Behind  the  club-shaped  gland  on  the 


yKW*2 


/»/»• 


a^. 


117 


•Ae 


Fig.  64.  —  Stage  in  which  the  external  apertures  of 
the  body,  prajoral   pit,  mouth,  first  gill-slit,  and   anus  g  ■    Wit', 

have  become  perforated.    Age  about  36  hours.     From 
the  left  side.     (After  Hatschek.) 

a/.  Alimentary  canal,     an.  Anus.     i.e.   Body-cavity.  pig.  64. 

i/i.  Notochord.    end.  Endostyle.    ^^i.  Club-shaped  gland, 
which    has    acquired    an    opening   to    the    exterior   on 

the  left  side  below  the  mouth.  ,.^^J'.  First  priiiiaiy  gill-slit.  >/i.  Mouth.  «.c.  Nerve- 
tube;  the  neurenteric  canal  has  closed  up,  but  the  nerve-tube  still  curves  r<jund 
the  hinder  end  of  the  notochord.  p/>.  Neuropore.  /<.o.c.  Pr;eoral  coelom  (rii;ht 
head-cavity),    p.p.  Pra;oral  pit  (left  head-cavity).    /.  i'rovisional  caudal  fin. 


F* 


ii8 


ni.vi:i.oi\yi:xr  of  ,lv/'///o.\c's. 


ri^ht  side  is  the  first  ^nil-slit,  openinj;  directly  to  the 
exterior.  At  the  time  of  its  actual  i)erf()ration  it  lies 
near  the  mid-ventral  line  of  the  body,  but  as  it  increases 
in  size  it  becomes  shifted  up  to  the  right  side. 

The  neurenteric  canal  is  closed  up,  and  the  nervc-tubc 
ends  blindly  behind,  being  curved  round  the  hinder  end  ot 
the  notochord.  Immediately  in  front  of  anil  below  the 
point  where  the  neurenteric  canal  formerly  existed,  the 
a>///s  has  now  made  its  appearance,  approximately,  if  not 
exactly,  in  ihc  mid-ventral  line*  (Fig.  64). 

We  will  now  return  to  consider  more  closely  the  exact 
development  of  the  mcsodermic  somites,  the  notochord, 
and  the  nerve-cord. 


Dcvelopvicut  of  Central  Ncnwus  System. 

As  in  the  craniate  Vertebrates,  so  in  Aniphioxus  the 
medullary  plate  arises  as  a  median  unpaired  longitudinal 
specialised  portion  of  the  dorsal  ectoderm.  The  way  in 
which  it  becomes  separated  from  t?e  superficial  ectoderm 
has  already  been  indicated  above,  but  it  can  best  be 
studied  in  transverse  sections. 

In  the  sections  shown  in  Figs.  65  and  66,  the  separation 
of  the  medullary  plate  from  the  ectoderm,  and  its  subse- 
quent conversion  into  a  closed  tube,  is  so  clearly  illus- 
trated, that  further  description  is  unnecessary.  A  unique 
feature  in  connexion  with  the  formation  of  the  central 
nervous  system  of  Aniphioxus  is,  that  the  medullary  plate 
sinks  below  and  becomes  covered  over  by  the  superficial 
ectoderm  before  it  takes  on  the  form  of  a  closed  tube,  so 
that  for  some  time  it  exists  as  a  half-canal  open  dorsally 

*  According  to  Hatschek,  the  anus  breaks  through  slightly  to  the  left  of 
the  micklle  line. 


EMli/<  1  ( 'A7C  DE I  'EL  O/WEXT. 


119 


H 


J 


I  i 


120 


DEVELOPMENT   OE  AMPHIOXUS. 


against  the  ectoderm.  Later  the  dorsal  margins  of  this 
half-canal  meet  and  fuse  in  the  middle  line,  and  so 
produce  the  medullary  tube  *  (Fig.  66). 

Origin  of  Mesoderm  and  Ccclom. 

In  consequence  of  the  flattening  and  incurving  of  the 
medullary  plate,  pressure  is  brought  to  bear  on  the 
dorsal  wall  of  the  archenteron,  and  the  dorso-lateral  bor- 
ders of  the  latter  acquire  the  form  of  two  longitudinal 
grooves  (Figs.  65  A  and  B).  It  is  from  these  grooves  that 
the  archenteric  pouches  are  split  off.  The  grooves  deepen, 
and  in  doing  so  become  divided  up  into  a  scries  of 
pouches.  Eventually  the  pouches  become  shut  off  from 
the  archenteron  gradually  from  before  backwards,  and 
then  appear  as  closed  cavities  on  either  side  of  the 
notochord,  which  has,  in  the  meantime,  been  developing 
(Fig.  65  F). 

In  the  higher  Vertebrates  the  mesoderm  arises  as  two 
solid,  lateral,  longitudinal  bands,  which  are  split  off  from 
the  primitive  endoderm.  These  mesodermic  bands  are  at 
first  unsegmented,  and  might  be  taken  to  correspond  with 
the  longitudinal  grooves  of  the  archenteron  of  Amphioxus, 
as  described  above.  Later,  only  the  dorsal  portion  of  the 
mesodermic  bands  undergoes  segmentation,  while  the 
vential  portion,  which  becomes  hollowed  out  to  form 
the  general  body-cavity,  is  never  segmented  in  the  crani- 
ate Vertebrates.  (Cf.  Fig.  33.)  In  Amphioxus  the  whole 
of  the  mesoderm  is  contained  in  the  archenteric  pouches, 
and  is,  therefore,  at  first  entirely  segmented. 

As  soon  as  the  pouches  have  lost  their  primitive  con- 

*  In  the  Ascidian  embryo  the  formation  of  the  medullary  tube  takes  place 
after  the  manner  typical  of  craniate  Vertebrates  (see  below,  I\'.). 


,;  s! 


EM  BR  YONIC  DE  VELOrMENT. 


121 


.n  c 


mye 


nexion  with  the  archenteron,  they  commence  to  extend 
dorsally  and  ventrally  between  the  ectoderm  and  the  in- 
ternal organs  (Fig.  66).  Meanwhile  the  cells  forming  the 
inner  or  visceral  wall  of  the  pouch  adjacent  to  the  noto- 
chord  elongate  transversely  and  longitudinally,  and  begin 
to  form  the  plate-like  muscle-fibres  of  the  myotome.  The 
cells  producing  these  fibres 
are  arranged  in  such  a  way 

that  ench  of  them  gives  rise  ^Sl^^^^^^-lm 

to  a  muscle-fibre  extending 
from  the  anterior  to  the  pos- 
terior limit  of  a  myotome.* 
The  muscles  are  at  first 
closely  approximated  to  the 
notochord  and  project  freely 

into  the  cavity  of  the  pouch.  Fig-  66.  — Transverse  section  througli 

tlie   middle   ot  tiie   Ijodv   of  an  emi)rvo, 

The  latter  gradually  grows  with  ten  pairs  of  somites,  to  show  the 

1  „     1  .-1     •,     ^  ^.  „    closure  of  medullarv  tube  and  tlie  dorsal 

downwards,    untd   it   meets  ^^^  ^,^,„,^^,  ^^^^^^  ^^  ^„^  mesodermic 
its  fellow  of  the  other  side  ;  somites,    (.^fter  hatsjhek.) 

al.  Alimentary  canal,    ch.  Notochord, 
the    two    fuse    together,    and    in  the  cells  of   which  vacuoles  liave  corn- 
so    the    cavity    is    made    con-    '"ence.l  to  form.    /..;/.  Commencing  for- 
-'  mation  of  lonyitudmal  muscle-plates  trom 

tinUOUS    from     side     to     side,     the  cells  forming  the   inner  wall  of  the 
.  .  somite,    iiiy.c.  .Myocuclomic  cavity. 

below  the  mtestine. 

When  this  occurs,  the  primarily  single  cavity  of  each 
archenteric  pouch  becomes  divided  into  two  portions ; 
namely,  a  dorsal  portion,  the  somite  proper  or  luyoavl, 
and  a  ventral  portion,  the  cccloni,  by  a  transverse  partition, 
which  arises  through  a  fusion  between  the  parietal  and 


*  Already  in  the  embryo  shown  in  Fig.  63  C,  and  even  at  a  somewhat  ear- 
lier stafje,  the  muscles  are  so  far  developed  that  the  body  can  be  bent  and 
jerked.  By  the  time  the  mouth  has  broken  through,  muscular  /oiomotion 
ciVectually  replaces  the  primitive  miliary  locomotion,  although  the  cilia  persist 
to  a  late  stage. 


(  ■   r 


I  22 


DEVELOPMEXr   OF  AMPHIOXUS. 


visceral  walls  of  the  cavity,  at  about  the  level  of  the  base 
of  the  notochord  (Fig.  67). 

The  dissepiments  between  the  myotomes  are  formed 
from  the  contiguous  walls  of  the  successive  pouches,  but 
ventrally,  in  the  region  of  the  coelom,  they  break  down, 
so   that   the   latter    then    becomes   a   continuous   unseg- 

mented  cavity.  On  account 
of  the  fact  that  the  archen- 
teric  pouches  give  rise  both 
to  the  cavity  of  the  somites 
{niyoccel)  and  to  the  general 
body-cavity  (coelom  proper 
or  splaiicJinococl),  they  are 
often  spoken  of  as  the  viyo- 
ccelomic  pouches.  The  cav- 
ity of  the  original  archen- 

Fig.  67.  _  Scheme   of   a   transverse    tcric     pOUChcS    is    knOWn    aS 

section    through    the    body   of    a    larva  j          pyij„ifr^„      ,ce/om,     the 

with   five  gill-shts,  to  show   the   division  •/                                         ' 

between     myocoil     and     splanchnocuil.  epithelial      Walls      of      which 
(After  Hatschkk.) 

n.c.    Spinal    cord    (medullarv    tube).  Constitute    the    lUCSodcrm. 

r>S.  Notochord.    Lm.  Muscles,     .y.  Myo-  ^^  differentiation  and   Or- 
coel.      sc.      Rudiment      of     sclerotome. 

(i/.  Alimentary  canal,    j./.f.  Sub-intestinal  ganOgCUy  prOCCCd,  the  mCSO- 

vein.    sp.  SplanchnoccEl.  .               . 

derm  gives  rise  to  (i)  the 
musciilaturcy  (2)  the  connective  tissue,  (3)  the  biood-vesse/s, 
(4)  the  reproductive  organs,  (5)  the  ccelomic  epitJielium  or 
lining  of  body-cavity,  also  called  the  peritoneu^n,  and 
(6)  the  excretory  tubules.  The  development  of  the  last- 
named  structures  has,  however,  not  yet  been  worked  out 
in  Amphioxus. 

The  parietal  layer  of  the  mesoderm  applies  itself  closely 
against  the  ectoderm,  and  gives  rise  to  the  cutis  of  the 
body-wall. 

The    connective    tissue-sheath   of   the    notochord   and 


W 


I  i!;'i 


EMBK  1  -ONIC  DE  J  'EL  0PM EXT. 


12.^ 


nerve-cord,  together  with  the  internal  sheath  or  fascia 
of  the  muscles  of  the  myotome,  arises  from  the  walls  of 
a  pouch-like  diverticulum  of  myocoel  which  grows  up  be- 
tween the  muscles  and  the  notochord  and  nerve-cord.  (Cf. 
Figs.  6"]  and  6%.)  The  myoccel  also  grows  downwards 
between  the  somatic  layer  of  the  peritoneum  and  the  ecto- 
derm (Fig.  68).  According 
to  Hatschek  the  dorsal  and 
ventral  fin-spaces  are  also 
derived  from  the  myocoel.^ 

The  diverticulum  of  the 
myocoel  which  has  just  been 
described  is  known  as  the 
sclerotome,  since  it  gives  rise 
to  the  fibrous  sheath  of  the 
notochord  and  nerve-cord, 
which  {i.e.  the  sheath)  in 
most  of  the  higher  forms 
becomes  replaced  by  carti- 
lage, and  finally  by  bone. 
In  the  craniate  Vertebrates       pjg,  gg.- Scheme  of  a  transverse 

the    sclerotome    arises    as    a    s^<=t'°"  through  region  between  atriopore 

and  anus,  of  a  young  Amphioxus  shortly 
solid     proliferation     of     cells    after  the   metamorphosis.     (After   Hat- 

from  the  visceral  wall  at  the  '''■'''^^l  dorsal  fin-space. 


■  my 


base  of  the  somite. 


my.  Myocii.'!. 
This    ^'■'  Sclerotome,    ao.  Aorta,     al.  Intotine. 
i.m.  Intercoelic  membrane,     s.t.v.  Sub-in- 
Solid       proliferation      is      Un-    testinal    vein.    sp.   Splanchnoca-l.     v.f.c. 

doubtedly  a  modification  of  ^''"^^''^'  '^"-^'"'^'='-'- 
a   hollow  diverticulum,    involving,    as    it    does,    only   the 
visceral    wall    of  the   somite,    precisely   as    we  find   it  in 
Amphioxus.'*     (Cf.  Fig.  33.) 

On  their  outer  surface  the  muscles  of  the  myotomes  are 
not  provided  with  a  sheath  of  connective  tissue  (fascia), 
standing,  in  this  respect,  in  contrast  to  the  condition 
which  obtains  in  the  Craniota. 


^--\m.-  Jyi 


^ 


m 


mr 


124 


DEVELOPMENT   OF  AMPJIIOXUS. 


%:v 


'S-^ 


1'' 


m 


Origin  of  the  Notochord. 

The  notochord  is  formed  from  the  endodermic  cells 
which  lie  between  the  mesodermic  pouchej.  and  constitute 
the  dorsal  wall  of  the  archenteron.  The  dorsal  wall  of 
the  archenteron  at  an  early  stage  becomes  converted  into 
a  shallow  longitudinal  groove  whose  concavity  is  turned 
towards  the  archenteric  cavity  (Fig.  65  D).  This  groove 
gradually  deepens  (Fig.  65  E),  and  eventually  its  walls 
become  closely  appressed  to  one  another  so  as  to  obliter- 
ate the  lumen  (Fig.  65  F).  Finally  the  adjoining  cells  of 
the  archenteric  wall  grow  across  the  gap  occasioned  by 
the  formation  of  the  notochord,  and  joining  together,  shut 
off  the  latter  from  any  participation  in  the  enteric  wall 
(Fig.  66).  In  this  way  is  the  notochord  separated  from 
the  endoderm  gradually  from  before  backwards.  Poste- 
riorly it  remains  for  a  considerable  time  fused  with  the 
endoderm  at  the  point  where  the  anterior  wall  of  the  neu- 
renteric  canal  beromes  continuous  with  the  dorsal  wall 
of  the  archenteron. 

We  have  indicated  above  that  the  differentiation  of  the 
notochord  takes  place  from  before  backwards.  At  its 
anterior  extremity  a  very  noteworthy  exception  to  this 
rule  is  prei:-ented,  In  the  region  of  the  first  myotome 
the  notochord  retains  an  open  communication  with  the 
archenteron  after  its  lumen  has  already  been  obliterated 
behind  this  point.  Moreover,  in  the  embryo,  with  eight 
pairs  of  myoccelomic  pouches  (Fig.  ^'^  bis),  the  front  end 
of  the  notochord  lies  some  distance  behind  the  front  end 
of  the  body,  while  the  anterior  portion  of  the  archenteron 
extends  beyond  the  notochord.  Eventually  the  notochord 
is  continued  to  the  front  end  of  the  body  by  becoming 
constricted  off  from  the  dorsal  wall  of  the  anterior  sec- 


EM  BR  \  WXR '  DE I  'EL  OPMEX  T. 


125 


tion  of  the  archenteron  in  the  usual  way.  This  retarded 
g'-ovvth  of  the  notochord  anteriorly  indicates  that  its  exten- 
sion to  the  tip  of  the  snout  is  a  secondary  phenomenon. 
Ancestrally  we  are  bound  to  assume  it  did  not  extend  so 
far  forwards.  The  forward 
extension  of  the  notochord 
is,  as  noted  above,  obviously 
useful  to  Amphioxus  in  ren- 
dering its  pointed  snout 
sufficiently  resistant  to  en- 
able it  to  burrow  in  the 
sand.  When  it  wants  to 
bury  itself  in  t-he  sand,  it 
has  not  to  take  pains  to  dig 
a  hole,  but  darts  in  in  the 
fraction  of  a  second. 

The  histological  differen- 
tiation of  the  notochord 
commences   soon  after   the 

sides  of   the   Chordal   groove         Fig-  68  <^».-Iv;..t)ryo  of  Amphioxus. 

°  with   eiglit  ]iairs  of  somites  to  show  the 

have  come  together  so  as  to    primary  relations  of  the  anterior  end  of 

the    notochortl.      From    above.      (After 

Hatschkk.) 


obliterate  the  lumen.     The 


cells     composing     the     notO-  P-'-   j'^ychordal    portion    of   archen- 

*            *^  teron,  which  becomes  converted  into  the 

chord     are,    at    the    first    ap-  head-cavities,  n.p.  Xeuropore.  ch.  Noto- 

.     .              11         r  chord;  over  which    hes  the  neural  tube. 

prOXimatlOn    of   the  walls    of  ,„y_  MyocL-lomic  pouches.    „e.  Neuren- 

the   groove,   placed  end   to  '^'"''^  ''■""'^'• 

^  N.B. —  In    this   and   other   figures   of 

end,  but  soon  begin  to  inter-    Amphioxus    embryos    here     reproduced 
,  ..  ,  ,,  after   Hatschek,   the    so-called    luesoder- 

lace  with  one  another  across  ,,,i,  ,,,3,,  ,,,,3  „,,,,  ,,,,,„  „,„j,„j  j,, 

the  middle  line    (Fi"".  (b^  F\     accordance    with    the    observations    of 

i-.-      0       h    Wilson  and  Lwoi'i'. 
and  finally  each  cell  comes 

to  occupy  the  whole  width   of  the  notochord  (Fig.  (y(:)). 

Meanwhile  vacuoles  begin  to  appear  in  the  cells  (Fig.  66). 

The  vacuolisatioii  of  its  component  cells  is  an  extremely 


120 


DEVELOPMENT   OE  AMPIIIO:':S. 


characteristic  feature  of  the  notochordal  tissue  throughout 
the  group  of  the  Vertebrates.     It  is  carried  on  to  such  an 

extent  in  Amphioxus  as  to 
obscure  the  original  cellular 
structure  of  the  notochord. 
The  cells  anastomose  with 
one  another  in  the  longitu- 
dinal direction,  and  so  pro- 
duce a  reticulum  the  meshes 


Fig.  69. -Median  sagittal  section  of  i>f  which  represent  the  vacu 

notochord  of    a    young    Ami)hioxus    nl    ^^^^^     ^^i\^osc     first     Oriirin 
8   mm.,  to  sliow  tlie  vacuolar  character 


IS 


of  the   notochordal   tissue   and   the  dis-    shoWU    in   Fig.  GG.       Most    of 
placement  of  the  nuclei  to  the  dorsal  and      ,  ,    .  ,     ^  ,, 

vemrai  borders.    (After  LwoFK.)  the  nuclci  bccomc eventually 

displaced  from  the  centre  of 
the  notochord,  and  are,  in  the  adult,  almost  exclusively 
confined  to  its  dorsal  and  ventral  aspects  (Fig.  69). 


»: 


T/ie  Pneoral  '^Head-cavities"  of  Ainphioxiis. 

Before  leaving  the  embryonic  period  of  the  development 
it  is  necessary  to  consider  the  origin  and  fate  of  what  may 
be  called  the  hcad-cavitics  of  Amphioxus  as  made  known 
to  us  by  the  work  of  Hatschek. 

They  arise  symmetrically  as  a  pair  of  diverticula  from 
the  anterior  portion  of  the  archenteron,  which  lies  at  first 
partly  in  front  of  the  notochor  d  (Fig.  68  bis)  and  completely 
in  front  of  the  myoccelomic  pouches  (Fig.  70). 

They  begin  to  appear  at  the  stage  in  which  some  eight 
pairs  of  pouches  are  already  present.  Their  origin  there- 
fore, in  point  of  time  and  the  subsequent  modifications 
which  they  undergo,  show  that  they  do  not  belong  to  the 
metameric  series  of  the  mesodermic  pouches,  but  are 
structures  sui  generis. 


EMBR  YOXIC  1)E  VELOPMKXT. 


127 


ra.d-i 


The  archentcron  extends  at  first  to  the  front  end  ot  the 
body.  Its  anterior  portion,  after  the  formation  of  several 
mesoblastic  somites,  becomes  marked  off  from  the  hinder 
region  by  a  slight  constriction,  which  gradually  becomes 
deeper  and  deeper  (Fig,  70),  until  finally  the  whole  of  this 
portion  of  the  archentcron  is  divided  into  two  separate 
sacs,  which  eventually  lose 
all  connexion  with  the  ar- 
chentcron ( Fig.  7 1 ).  The  ali- 
mentary canal  now  no  longer 
reaches  to  the  anterior  ex- 
tremity of  the  body.  Very 
soon  after  their  separation 
from  the  archentcron  these 
sacs  enter  upon  a  series  of 
changes  by  which  their  origi- 
nally symmetrical  disj^osi- 
tion  is  entirely  destroyed. 

Already  in  Fig.  71  it  can 
be   noticed    that    the    right 

sac   is  becoming   larger  than  pig.  70. -Embryo,  with  nine  pairs  of 

the  left,  and  the   epithelium    primitive  somites  seen  in  optical  section 

^  from   the  ventral    snrtace,    to   siiow    th'j 

lining  its  walls   is    losing    its    origin  of  the  head-cavites.     (After  H.vi- 

original    cubical    character,       ^^^.d.  Right  head-cavity.   i.a.d.  Left 

the    inner    ends   of    the    cells    liead-cavity.    w>'.f.  Myoco^-lomic  pouelus 

(primtive  somites),    ^•>c.  Arclienteron. 

are  rounding  off,  and  in  fact 

it  is  being  converted  from  a  cubical  to  a  flattened  pavement 
epithelium  (Figs.  6^^  C  and  64).  The  left  sac,  on  the  con- 
trary, retains  its  original  form  and  dimensions  for  a  long- 
time. During  the  asymmetrical  changes  affecting  the  two 
sacs,  which  take  place  coincidently  with  the  formation  of 
the  snout,  the  left  one  comes  to  lie  transversely  below  the 
notochord,  while  the  right  sac  becomes  greatly  enlarged 


.^i.  \ 


128 


DEVELOPMENT  OF  AMP/I /OX  US. 


'"€ 


! 


I! 

(1 

I.' 

•  li. 
f 

t  i' 

1 

II;    i! 


Ill-  'H 


ra  «? 


CJ.^ 


l.a<f 


and  constitutes  the  cavity  of  the  snout  lying  below  the 
notochord  (Fig.  63  C). 

Shortly  after  the  breaking  through  of  the  mouth  the 
left  sac  acquires  an  opening  to  the  exterior  on  the  left  side 
of  the  body  (Fig.  64).  The  right  sac  becomes  thQ  fraornl 
body-cavity  or  coelom  of  the  '  head,"  while  the  left  sac  is 
known  as  the  pnvoral  pit.  It  is  necessary  to  emphasise 
the  fact  that  these  two  structures  which  are  so  different 

in  their  fully  formed  con- 
dition are  at  first  perfectly 
similar  and  symmetrical  and 
form  a  pair  of  "head-cavi- 
ties." Ultimately,  as  we 
have  seen,  only  one  of  them 
actually  persists  as  a  head- 
Fig.  71. —  Anterior  portion  of  em-  cavity ;  namely,  the  right  one. 

brvo,    witii    thirteen    primitive    somites,  nni  .  •  •  r 

from  the  ventral  side  in  optical  section!  ^hc   entire    COnVCrSlOn    of 

(After  hatschkk.)  the  left  sac  into  the  prreoral 

r.a.d.  and  l.a.d.  Right  and  left  head-       .     . 

cavities,  ci.g.  Rudiment  of  club-shaped  pit  IS  probably  to  be  regarded 
^''^"'"  as  a   secondary  or   cenoge- 

netic  phenomenon,  but  the  acquirement  of  an  opening  to 
the  exterior  is  probably  not  secondary,  since  a  smiilar 
opening  (the  proboscis-pore)  occurs  in  Balanoglossus. 

Tn  addition  to  the  above-described  peculiarities  which 
sufficiently  distinguish  the  head-cavities  from  the  myocoe- 
lomic  pouches,  must  be  mentioned  the  fact  that  at  no  point 
of  their  epithelial  walls  are  muscles  developed. 

It  is  probable  that  the  prceoral  head-cavities  of  Amphi- 
oxus  are  homologous  with  the  prcemandibular  cavities  of 
the  higher  Vertebrates,  from  the  walls  of  which  the  greater 
number  of  the  eye-muscles  are  developed.*     This  view  is 

*  This  is  also  the  opinion  of  Kupffcr.  Singularly  enough  van  Wijhe  has 
advanced  the  view  that  only  the  right  head-cavity  of  Amphioxus  is  to  be 


KM  BR  \ '( K\7C  /)/■:  1 1:1.  Or.MEXT. 


129 


Strongly  confirmed   by  the  mode  of  development  of  the 
prnemandibular  cavities  in  the  Cyclostomes. 

In  these  fishes,  vox  Kupf- 
FER  has  shown  that  they 
actually  appear  in  the  form 
of  a  pair  of  diverticula  from 
the  anterior  extremity  of 
the  archenteron  (Fig.  ']2). 
If  a  comparison  be  made 
between  Figs.  70  and  72,  it 
will  be  at  once  manifest  how 
close  the  correspondence  is 

Fig.   72.  —  Horizontal    projection   of 

between     the     modi'     of     de-    pharynx  and  praoral  endockrniic  exten- 

1  .       c    .\       \  y  ■      sion  of  a  vounir  .Immoca-tes  planen    of 

Velopment    of    the  head-CaVl-    3.^  „„,^     reconstructed  from  a  STics  of 

ties    in    AmphioXUS     and     in    transverse  sections.     (After  Kll'KKK.) 

p.e.    I'r.uoral     endodermic    e  ^tension 

(pnvorale  Endodernitasche).  /;;.  and  m. 

I'ra-niandibular  and  niantlilniU).'  portions 

avities.    ph.  Cavity  of  pharynx. 

/,  2,  J.  First  three  jiairs  of  gill-pouches. 

N.B. —  KujifTer  considers  it  j)robable 
that  the  mandibular  as  well  as  the  pr;e- 
mandibular  cavities  arise  from  tlie  single 
pair  of  endodermic  diverticula.  In  the 
course  of  the  following  pages  I  have 
referred    chiefly   to    the    pr;vmandibular 


Ammocoetes.      In    the    Se 

1        I    .  .1  •       •!        *j.  •         I  i;eiiiaiiii 

lachians    the    simdarity    is  ^f  iiead-c 
hardly  less  striking.** 

Endostylc  and  Pigvictit 
Grannies. 


In  Fio".  64  there    is    to    be    cavities  alone  so  as  to  avoid  complica- 
^  tions. 

noticed   a  vertically  placed 

structure  lying  in  front  of  and  contiguous  with  the  club- 
shaped  gland.  It  is  a  tract  of  very  high  cylindrical  cells 
forming  part  of  the  right  wall  of  the  alimentary  canal  in 


!l 


homologised  with  the  praimandibular  cavity  (see  below,  V.).  Kupffer  re<;arils 
the  privmandibular  and  mandibular  heaci-cavities  as  rudimentary  or  meta- 
morphosed gill-pouches.  This  deduction  is  entirely  foreign  to  the  standpoint 
which  I  have  adopted.  The  conclusion  may  seem  plausible  from  the  con- 
ditions observed  in  Acipenser  alone;  but  when  these  are  regarded  from  a 
comparative  point  of  view,  the  deduction  is.  to  my  mind,  unjustified.  It  should 
be  added  that  Kupffer  has  shown  that  the  head-caviHes  (pritmandibular  and 
mandibular)  of  Acipenser  also  arise  as  endodermic  pouches. 


w 


:?,o 


DEVELOPMEXr   Of  .lM/'J/JO.\iS. 


this  region.  (Cf.  Figs.  65  (7  and  75.)  I  have  shown  that 
this  epithelial  tract  is  the  rudiment  of  the  cndostyle  {vide 
infra). 

It  is  a  curious  fact  that  the  first  trace  of  pigment  to 
appear  in  the  nerve-tube  is  not  the  eye-spot,  but  that  at  a 
constant  point  in  the  region  of  the  fifth  somite  a  black 
pigment-spot  is  deposited  in  a  cell  in  the  ventral  wall  of 
the  medullary  tube.  This  is  followed  by  another  smaller 
pigment  granule  slightly  posterior  to  the  first  (Fig.  6T)  C ). 
The  eye-spot  appears  at  the  end  of  the  embryonic  period. 


t 


'  m 


> ' 


.    .     ,   .  LARVAL   DEVKLOPMENT. 

Formation  of  Primary  Gills/its,  etc. 

With  the  establishment  of  the  definite  relations  oi  the 
head-cavities,  the  mouth,  club-shaped  gland,  first  gill-slit, 
and  anus,  the  embryo  enters  upon  the  larval  phase  of  the 
development. 

It  is  no  longer,  or  only  very  rarely,  to  be  taken  from 
the  surface  of  the  sea,  but  descends  to  a  depth  of  several 
fathoms.  When  kept  in  aquaria,  the  larvae  can  often  be 
observed  to  be  suspended  vertically,  and  apparently  quite 
motionless  in  the  water.  This  suspension  is,  no  doubt; 
effected  by  the  movement  of  the  long  cilia,  or  flagella, 
with  which  the  ectoderm  is  provided,  each  cell  possessing 
one  flagellum.^ 

The  principal  changes  which  take  place  during  the  early 
stages  of  this  phase  of  the  development  are  the  addition  of 
new  myotomes,  the  formation  of  new  gill-slits,  in  meta- 
meric  order,  in  an  unpaired  series  on  the  right  side  of  the 
larva,  to  the  number  of  from  twelve  to  fifteen,  or  even 
sixteen  (the  more  usual  number  being  fourteen),  and  the 
origin  of  the  atrial  cavity. 


II 


LAR I :U.   DE VELOrMEXT. 


131 


Each  gill-slit  breaks  through  in,  or  slightly  to  the  right 
of,  the  mid-ventral  line,  and  then  grows  well  up  on  the 
right  side  of  the  body.  A  larva  with  three  gill-slits  and 
the  indication  of  a  fourth  is  represented  in  Fig.  73.  The 
originally  circular  mouth  has  grown  to  a  much  larger  size, 
and  extends  on   the  left  side  anterior  to   the  endostylar 


end 


an 


Fig-  73-  —  l^iirva  of  Amphioxus,  with  three  gill-slits  and  the  rudiment  of  a 
fourth  ;  from  the  left  side.     (After  Lankksi'KR  and  W'lI.i.EV.) 

p.p.  Pra;oral  pit.  end.  Endosfyle  lying  on  right  side,  seen  tlirough  the  wide 
lateral  mouth,  gl.  Position  of  external  aperture  of  club-shaped  gland.  p.s' .  First 
primarv  giil-slit.    an.  Anus. 

M. 13.  — Actual  length  of  larva,  nearly  iVa  mm. 


tract  (which  is  on  the  right  wall  of  the  pharynx)  and 
posterior  to  the  first  gill-slit.  The  oral  opening  later 
attains  to  relatively  gigantic  dimensions,  and  forms  one 
of  the  most  striking  features  of  the  larva. 

The  anus  is  now  displaced  from  its  original  ventral 
position  to  the  left  side  in  consequence  of  the  increased 
development  of  the  provisional  caudal  fin.  The  latter 
consists  of  elongated  ectodermal  cells,  in  which  a  certain 
amount  of  brown  pigment  is  deposited.  Later,  when 
the  dermal  expansion,  which  has  been  described  above  as 
the  definitive  caudal  fin,  begins  to  grow  out,  it  pushes  the 
cells  composing  the  provisional  fin  before  it,  so  that  they 
form  a  fringe  round  its  border.  Eventually  the  provisional 
fin  disappears  entirely. 

The  gill-slits   now  go  on  adding  to  their  number,  one 

after  the  other,  until  the  larva  reaches  the  stage  shown  in 

74.     In  this  larva  there  are  fourteen  primary  unpaired 

ill-slits,  lying,  for  the  most  part,  on  the  right  side  of  the 


Fig 


qii|  i^iimfH 


( 


i*'^ 


132 


DI-.n-./.O/'MJ.XJ-   Of  AMJ'J//i>.\(S. 


pharynx,  although  the  more  posterior  slits  bend  under  the 
pharynx,  while  the  most  posterior  have  a  median  ventral 
position. 

In  front  the  f;ill-slits  still  open  directly  to  the  exterior, 
but  the  rij^ht  metapleural  told  is  seen  to  be  hanginj,^  over 
the  tops  of  them  ;  while  the  hinder  slits  now  open  into 
the  partially  formed  atrium,  which   has  alreatly  closed  in 


r  ■ 

I'' 


'.!'  '9 


Fig.  74.  —  Anterior  portion  of  l;irva,  with  fourteen  primary  gill-slits  and  rudl- 
ments  of  the  secondary  gill-slits;  viewed  as  a  transparent  object  from  the  right 
side.     (After  WlLI.KV.) 

.f.d.  Sense-organ  of  (invoral  pit  (groove  of  Hatschtjk).  e.  Kndostyle.  ffl.  In- 
ternal o]5ening  of  chib-sha|)e(l  glantl.  s..s.  Rudiments  of  secondary  gill-slits.  ;*.i'', 
/.('^.  Thirteenth  and  fourteenth  jirimary  gill-slits.  The  lower  margin  of  the 
mouth  is  seen  through  the  anterior  gill-slits. 

Total  length  of  larva,  nearly  3*2  mm. 

posteriorly,  as  described  above.  The  larva  is  remarkably 
transparent,  so  that  its  internal  organisation  can  be  seen 
as  clearly  as  possible  through  the  outer  body-wall. 

The  long  axis  of  the  primary  gill-slits  is  approximately 
at  right  angles  to  the  long  axis  of  the  body.  They  are 
not  more  numerous  than  the  myotomes  in  the  correspond- 
ing region  of  the  body,  so  that  the  hranclnomcry  at  this 
stage  coincides  with  the  muscular  vietaviery.  In  Fig.  'ji 
the  first  gill-slit  was  somewhat  larger  than  the  second  and 
third.  At  about  that  stage,  however,  its  further  growth 
became  arrested,  and  now  it  is  seen  to  be  considerably 
smaller  than  those  which  immediately  follow  it. 

In  addition  to  its  external  opening  on  the  left  side,  be- 


5r<v- 


IAJ<  l  AL  DE I  'EL  0PM EXT, 


^11 


~tm 


tXC-' 


end 


r./n 


Fig-  75-  ~  Transverse  sections  through  the  region  of  the  mouth  of  larva-  ^f 
Amphioxus,  to  stunv  the  endostyle  .ind  the  external  and  internal  openings  of  i.iub- 
shaped  gland.     (After  Lankkstek  and  W'ii.i.ky.) 

A.  Section  passing  through  the  anterior  corner  of  the  mouth  of  a  larva,  with 
eleven  gill-slits. 

/i.  Section  passing  through  the  middle  of  the  mouth  of  a  larva,  witli  twelve 
gill-slits. 

al.  Pliaryngeal  cavity,  b.c,  Coelom  (spjanchnocoel).  br.e.  Branchial  epithelium. 
e.a.  Branchial  artery,  end.  Endostyle.  ex.o.  External  opening  of  club-shajied 
gland,  f.c.  Dorsal  fin-space.  .^V.  Lower  portit)n  of  club-shaped  gland.  ,;'..t'.  First 
gill-slit.  t.m.  Intercoelic  membrane,  in.o.  Internal  opening  of  clut)-shaped  gland. 
l.d.  Left  aorta;  there  is  no  corresponding  right  aorta  in  the  larva,  tn.  Mouth. 
'■./;/.  Rudiment  of  rigtit  metapleur;  a  mere  ectodermic  thickening  in  .-/ ;  a  solid 
thickening  of  the  cutis  in  A',  in  which  two  of  the  original  enlarged  ectoderm  cells 
liave  become  imbedded.  These  curious  cells  occur  over  a  long  stretch  of  the 
metapleural  folds  during  this  phase  of  the  development,  disappearing  eventually. 

In  li,  the  left  metapleur  is  indicated  by  an  ectotlerniic  thickening  immediately 
below  the  gill-slit.     .v.  So-called  nephridium  of  Hatschek. 


.■^ 


134 


DEVELOPMENT   OF  AMPHIOXUS. 


,!,! 


low  the  mouth  (see  Fig.  64),  the  club-shaped  gland  has 
now  acquired  an  op*^  \ing  at  its  upper  extremity,  on  the 
right  side,  into  the  pharynx."  The  gland  lies,  as  usual, 
behind,  and  closely  approximated  to,  the  endostylar  tract, 
which  is  bent  forwards  on  itself  at  its  upper  end  (Figs.  75 
A  and  B). 

Pigment-spots  are  present  in  great  numbers  at  the  base 
of  the  neural  canal.     The  pigment  is  deposited  in  special 


n.e ■/ 


loh 


(After 


ax-nt  arm. 

Fig.  76. — Transverse  sections  through  the  region  of  the  prteoral  pit. 
Lankf.stkr  and  WU.I.KY.) 

A.   Through  a  larva,  with  twelve  gill-slits  and  no  atrium. 

li.  Through  a  larva,  in  which  the  atrium  was  closed  in  over  all  the  gill-slits 
except  the  lirst  two.     (Cf.  Y\^.  38  C) 

a.r.m.  Anterior  median  jiortion  of  right  metapleur.  p.o.c.  Prasoral  body-cavity 
(right  head-cavity)  ;  this  cavity  becomes  much  reduced  after  the  metamorphosis, 
and  is  largely  filled  up  by  gelatinous  tissue,  p.p.  Pr.toral  pit.  s.o.  Sense-organ 
of  priT'oral  pit  (groove  of  H.'itschek).  l.o.h.  Rudiment  of  left  half  of  oral  hood. 
vty  .  Sclerotome  (diverticulum  of  myococl  my).     Other  letters  as  above. 

Section  li  is  taken  through  a  plane  slightly  posterior  to  section  A. 


LAR I  'AL   DE I  'EL  OPMENT, 


i35 


cells,  the  pigment-cells,  which  arise  as  modified  epithelial 
cells  of  the  central  canal.  These  cells  send  out  several 
branching  processes,  which  lose  themselves  in  the  fibrous 
tract  of  the  spinal  cord. 

Already  in  the  youngest  larva  —  namely,  that  shown  in 
I'^ig.  64  —  the  praioral  pit  had  become  subdivided  into  two 
portions,  which,  however,  retained  a  free  communication 
with  one  another. 

In  the  course  of  the  changes  which  the  left  head-cavity 
had  to  undergo  in  its  conversion  into  the  prccoral  pit  it 
had  come  to  lie  transversely  below  the  notochord.  Sub- 
sequently it  extended  itself,  in  the  form  of  an  offshoot, 
dorsally  to  the  right  of  the  base  of  the  notochord. 

This  offshoot  from  the  prreoral  pit  appears  to  serve  as  a 
special  sense-organ  lying  ultimately,  as  mentioned  above, 
in  the  roof  of  the  oral  hood,  whose  function  is  possibly  to 
test  the  water  as  it  enters  the  mouth  (Figs.  76  A  and  B, 
and  Fig.  74,  etc.). 


ter 

lits 
'ity 
:an 

3d. 


Fotination  of  Secondary  Gill-slits. 

Above  the  primary  gill-slits  in  Fig.  74,  and  like  them,  on 
the  right  side  of  the  body,  is  to  be  observed  a  longitudinal 
ridge  nrovided  with  a  series  of  nodal  enlargements  which 
alternate  with  the  primary  gill-openings,  the  first  of  them 
lying  above  and  between  the  third  and  fourth  primary  slits. 
Each  of  these  enlargements  represents  a  thickening  in  the 
wall  of  the  pharynx,  which  has  undergone  fusion  with  the 
body-wall  beneath  the  right  metapleural  fold,  in  the  angle 
formed  by  the  latter  with  the  body-wall. 

These  metameric  fusions  of  the  pharyngeal  wall  with  the 
body-wall  are  the  forecast  of  a  second  row  of  gill-slits,  whose 
relation  to  the  primary  row  will  become  clear  as  we  pro- 


If: 


1^,6 


DEVELOrMEXT   OF  AMPHIOXUS. 


ceed.  With  their  appearance,  the  larva  enters  upon  that 
phase  of  its  development  which  has  been  called  the  later 
larval  period.  It  is  the  period  of  the  metamorphosis  of 
the  larva,  during  which  the  pronounced  asymmetrical 
arrangement  of  the  parts  is  exchanged  for  the  partial,  but 
not  absolute,  symmetry  which  we  have  noted  in  the  adult. 
The  metamorphosis,  therefore,  consists  largely  in  the  sym- 
metrisation  of  the  larva. 

The  simultaneous  appearance  of  the  six  nodal  thicken- 
ings in  the  exact  position,  shown  in  Fig.  74,  is  very 
constant.  Shortly  afterwards  a  minute  perforation  appears 
in  the  centre  of  each  thickening  almost  simultaneously, 
except  in  the  case  of  the  first,  which  usually  becomes 
perforitted  rather  later  than  the  others.  The  originally 
small  circular  openings  of  the  secondary  gill-clefts  gradually 
increase  in  size  and  become  oval  in  shape,  their  long  axes 
being  parallel  to  the  long  axis  of  the  body,  instead  of  at 
right  angles  to  it  as  in  the  case  of  the  primary  slits. 

Next,  the  upper  borders  of  the  secondary  slits  begin  to 
flatten,  and  later  to  show  signs  of  curving  downwards. 
The  changes  in  shape,  which  affect  the  secondary  slits  at 
the  stages  now  under  consideration,  may  be  expressed  by 
saying  that  they  are  at  first  shaped  like  a  biconvex  lens, 
then  like  a  plano-convex  lens  with  the  flat  surface  directed 
upwards  and  the  convex  surface  downwards,  and  finally 
like  a  concavo-convex  lens  with  the  concavity  directed 
upwards  (F'ig.  ']']). 

During  these  changes,  which  do  not  take  place  in  all  the 
secondary  slits  at  the  same  time,  the  last  one  especially 
retaining  for  a  long  time  its  primitive  shape,  the  walls  of 
the  successive  slits  become  sharply  rounded  off  and  distinct 
from  one  another,  and  anew  perforation  makes  its  appear- 
ance in  front,  above,  and  between  the  second  and  third 


LAK VAL  DE VELOPMEXT. 


13; 


primary  slits.  This  new  slit  constitutes  the  definitive  first 
slit  of  the  secondary  series  (Fig.  "]"]). 

The  larva  shown  in  Fig.  "jj  presents  a  very  different 
aspect  from  that  shown  in  Fig.  74 ;  the  transition  from  one 
stai;e  to  the  other  is,  of  course,  gradual,  and  all  intermediate 
steps  can  be  observed.  In  the  stage  which  we  are  now 
considering  (Fig.  ']'j\  the  atrial  cavity  has  become  com- 
pletely closed  up  in  front,  so  that  now  none  of  the  gill-slits 
open  direcfrly  to  the  exterior. 

None  of  the  primary  slits  now  lie  entirely  on  the  right 
side,  but  they  have  become  bent  under  the  pharyn.x,  and 


Fig.  77.-  -Anterior  portion  of  larva,  in  which  the  secondary  slits  have  become 
perforated,  and  the  primary  slits  are  passing  round  to  the  left  side.  From  the  right 
side.     (After  WiLLF.Y.) 

5,0.  Senre-organ  of  prreoral  pit,  v.  Right  half  of  velum,  e.  Endostyle,  grow- 
ing beyond  the  club-sha]H'tl  gland  gl.  p.s  ,  First  primary  slit,  much  reduced  in 
size.  s.s' .  First  secondary  slit.  p.s\^.  Twelfth  primary  slit,  behind  which  is  to  be 
seen  a  vestige  of  the  thirteenth  slit. 


thus  extend  round  to  the  left  side.  This  bodily  migration 
of  the  primary  slits  from  one  side  to  the  other  occurs  in 
correlation  with  the  increase  in  size  of  the  secondary  slits, 
which,  as  they  continue  to  grow,  push,  as  it  were,  the 
primary  slits  before  them,  and  so  cause  the  latter  to  bend 
under  the  pharynx  in  the  way  described.  The  peculiar 
growth  by  which  the  primary  gill-slits  are  gradually  carried 
from  the  right  to  the  left  side,  may  be  described  as  a  trans- 
verse or  rotatory  growth  affecting  the  pharynx  ///  toto  in 


(J^ 


1- 


'sm 


Ml 

11   -'I' 


138 


DEVEI.OrMEXT   OF  AMPIIIOXiS. 


the  region  of  the  secondary  slits.  Such  of  the  primary 
slits  as  occur  behind  this  region  are  not  affected  by  the 
rotatory  method  of  growth,  and  retain  their  original  position 
in  the  mid-ventral  line  of  the  pharynx. 

It  is  to  be  noted  also  that  there  are  only  twelve  primary 
gill-slits  at  this  stage.  Assuming  that  in  the  particular 
larva  here  figured  there  were  originally  fourteen  primary 
slits,  the  fourteenth  has  closed  up  and  vanished  without 
leaving  a  trace,  while  a  vestige  of  the  thirteenth  can  still 
be  recognised.  The  actual  process  involved  in  the  closure 
and  disappearance  of  a  certain  number  of  the  primary  slits 
can,  as  we  shall  see,  be  readily  observed  in  the  living  larva. 

Cliib-sliapcd  Gland  and  Endosiyle. 

The  internal  aperture  of  the  club-shaped  gland  into  the 
pharynx  is  exceptionally  plain  at  this  stage,  and  its  refring- 
ent  walls  and  relatively  large  size  give  it  a  curiously  slit- 
like appearance.  We  shall  find  that  the  gland  subsequently 
atrophies,  but  the  most  persistent  part  of  it  — that  is  to  say, 
the  last  part  of  it  to  disappear  —  is  precisely  the  internal 
opening  with  its  refringent  border. 

The  cndostyle,  whose  primary  position,  as  we  have  seen, 
was  immediately  in  front  of  the  club-shaped  gland,  now 
presents  a  remarkable  condition.  It  has  begun  to  grow 
backwards  and  downwards,  being  probably  pulled  down, 
so  to  sjieak,  by  the  general  rotatory  growth  of  which  we 
have  spoken  above ;  and  so  the  club-shaped  gland  no 
longer  lies  behind  it,  but  upon  it.  The  gland  itself  being 
disconnected  with  the  wall  of  the  pharynx,  except  at  its 
upper  end  where  it  opens  into  the  latter,  is  not  affected 
by  the  complicated  changes  to  which  the  pharyngeal  wall, 
including  gill-slits,  mouth,  and  cndostyle,  is  subjected,  so 


LA  R I  \l  I.   DE I  'EL  OEM  EX  1 '. 


139 


that  it  forms  a  convenient  punctuui  fixnm  with  relation  to 
which  the  growth  of  neighbouring  structures,  particularly 
that  of  the  endostyle,  can  be  determined. 

The  upper  and  lower  limbs  of  the  endostyle  are  inclined 
to  one  another  at  an  acute  angle,  and  may  be  said  to  form 
two  unequal  sides  of  a  triangle,  the  apex  of  which  is 
directed  backwards  between  the  rows  of  secondary  and 
the  primary  gill-clefts  (Fig.  ^^)> 

Between  the  two  rows  of  slits  on  the  right  side  of  the 
body  there  is  a  blood-vessel,  representing  the  anterior 
continuation  of  the  sub-intestiwal  vessel,  which  ends  blindly 
in  front  above  the  first  primary  slit.  This  is  the  future 
ventral  branchial  artery,  with  which  we  are  already  ac- 
quainted. When  its  final  situation  in  the  mid-ventral  line 
below  the  endostyle  is  remembered,  its  position  in  the 
larva  high  up  on  the  right  side,  as  in  Fig.  74,  will  appear 
very  striking. 

Continued  Migration  of  Primary  Gill-slits. 

The  secondary  slits  now  go  on  growing  in  size,  and  the 
primary  slits  gradually  tend  to  disappear  entirely  from  the 
right  side  until,  as  in  Fig.  "^"^j  only  the  original  upper  por- 


in    /IS 


Fig.  78.  —  Antrrior  portion  of  larva  from  right  sitic,  to  show  the  backwartl 
Rruwth  of  the  endostyle  between  the  primary  and  secondary  gill-slits.  (After 
Wii.i.KV.) 

.f.(>.  Sense-organ  of  prn'oral  pit.  p.s'.  First  ])riniary  slit.  in.  Internal  opening 
of  eliib-shaped  gland.     <•.  iMulostyle,     /./>.  Peripharyngeal  ciliated  band. 


f^ 


i'ili 


.%:     •!,' 


140 


DEVELOrMKXT   OF  AM  PHI  OX  CS. 


tions  of  them  are  visible  from  this  side.  In  some  of  the 
secondary  slits  the  dorsal  margin,  which  had  previously 
begun  to  curve  downwards,  has  now  reached  the  ventral 
margin  and  fused  with  it  (Fig.  78,  third  secondary  slit). 
In  this  way  is  the  tongue-bar  formed,  and  the  primitively 
simple  gill-opening  is  divided  into  two  distinct  halves. 
The  formation  of  the  tongue-bars  occurs  in  the  secondary 
slits  considerably  in  advance  of  the  primary,  both  actually 
and  relatively,  since  the  latter  have  existed  all  through  the 
earlier  period  of  the  larval  development  without  a  trace  of 
tongue-bars. 

PcripJiaryngeal  Ba?ids. 

The  endostyle  has  now  grown  a  long  distance  behind 
the  club-shaped  gland,  and  extends  backwards  between 
the  two  rows  of  slits  as  far  as  the  middle  of  the  second 
secondary  slit.  From  the  anterior  part  of  the  upper  half 
of  the  endostyle,  which  is  now  nearly  equal  in  length  to 
the  lower  half,  arises  an  epithelial  tract  in  the  wall  of  the 
pharynx,  which  appears  in  the  form  of  a  band  of  ciliated 
cells,  and  proceeds  backwards  below  the  notochord  to  the 
end  of  the  pharynx.  A  corresponding  ciliated  band  occurs 
in  the  left  wall  of  the  pharynx,  proceeding  from  a  similar 
point  in  the.  lower  limb  of  the  endostyle.  In  their  course 
below  the  notochord  the  two  bands  take  part  in  forming 
the  hyperpharyngeal  (dorsal)  groove  of  the  pharynx. 

Atrophy  of  First  Primary  Gill-slit  and  Club-shaped 

Gland,  etc. 

We  have  already  seen  indications  of  a  reduction  in  the 
size  of  the  first  primary  slit.  This  reduction  has  advanced 
considerably  in  the  stage  we  are  now  describing  (Fig.  78), 
where  the  slit  in  question  is  only  recognisable  in  side 
view  as  a  small  groove. 


LAR  J  'AL   DE  VEL  OPMEXT. 


141 


The  next  stage  to  be  considered  is  characterised  above 
all  by  the  simultaneous  atrophy,  closure,  and  disappearance 
of  the  club-shaped  gland,  and  the  first  primary  gill-slit 
(Fig.  79).  At  this  stage  the  increase  in  size  of  the 
secondary  slits  has  progressed  to  such  an  extent  that  the 
primary  slits  have  been  displaced  entirely  from  their 
original  position,  and  are  no  longer  to  be  seen  from  the 


Fig.  79.  —  Anterior  portion  of  larva  from  right  side  after  the  disappearance  of 
the  club-shaped  gland.     (After  Wu.LEY.) 

s.o.  Sense-organ,  e.  Endostyle.  p.b.  Peripharyngeal  band,  s.s' .  First  secondary 
slit. 

right  side,  except  in  the  case  of  the  hindermost  slits  of 
the  series,  which  remain,  as  mentioned  above,  in  a  median 
ventral  position  until  their  disappearance. 

A  larva  seen  from  below,  so  as  to  show  the  relative 
positions  of  the  gill-slits  and  endostyle,  etc.,  at  this  stage, 
is  represented  in  Fig.  80. 

It  is  obvious,  from  what  has  been  said  above,  that  in  the 
passage  of  the  primary  slits  from  their  original  position  on 
the  right  side  of  the  body  to  their  final  position  on  the  left 
side,  their  dorsal  and  ventral  margins  are  reversed.  What 
was  at  first  the  dorsal  edge  of  a  primary  slit  becomes  its 
ventral  edge,  and  vice  versa.  In  other  words,  what  is 
actually  the  dorsal  border  of  the  primary  slits  in  Fig.  74 
is  viorpJiologically  the  ventral  border  ;  and  conversely,  wliat 
is  actually  the  latter  is  morphologically  the  former  ;  and  it  is 


142 


DEVEI.orMIiXT   OF  AMPIIIOXUS. 


from  the  latter,  towards  the  completion  of  the  rotatory 
growth,  which  carries  the  slits  from  one  side  to  the  other, 
that  the  tongue-bars  arise  (Fig.  80). 

The  vertical  and  longitudinal  axes  of  most  of  the  slits, 
both  primary  and  secondary,  are  now  almost  equal ,  but 
the  original  difference  in  this  respect,  which  we  noted 
above,  is  still  to  be  observed  in  the  case  of  the  foremost 
and  hindmost  slits  of  the  two  series.  (Cf.  Fig.  80,  s.s^ 
and  pJ-y  and  s.s^  and  p.s^^.)     The   first  primary  slit  has 


Fig.  80. — Anterior  portion  of  larva  of  same  age  as  in  Fig.  79,  seen  from  the 
ventral  surface.     The  pharynx  is  ffattened  out.     (.After  WlLI.KY.) 

cli.  Xotochorfl.  w.  Entrance  to  mouth,  v.  Velum,  p.s"^.  Vestige  of  first 
primary  slit.  p.s'-.  Secondary  primary  slit.  /.,ii".  Tenth  jirimary  slit.  p.s^-.  \'es- 
tige  of  twelfth  primary  slit.  s.s^.  First  secondary  slit.  e.  Endostyle.  s.s.  Eighth 
secondary  slit.     a.  Atrium,  pressed  aside. 

now  completely  closed  up,  and  its  former  existence  is 
barely  indicated  by  a  loose  granular  appearance  at  the 
place  it  formerly  occupied. 

The  alternation  of  the  gill-slits  of  the  two  series  comes 
out  very  clearly  in  Fig.  80.  In  most  of  the  secondary 
slits  the  formation  of  the  tongue-bars  is  completed  ;  but 
not  so  in  any  of  the  primary  slits,  where  it  is  only  be- 
ginning. 

There  are  now  eight  secondary  slits,  an  additional  one 
having  been  added  behind,  alternating  with  the  ninth  and 
tenth  primary  slits.  Usually  the  formation  of  secondary 
slits  stops  at  this  point,  no  more  being  formed  until  the 


LAK I •. //,    DK 1 1:1. OIWIEXT. 


143 


number  of  primary  slits  is  reduced  to  the  same  number  ; 
namely,  eight. 

Since  it  is  usual  for  the  primary  slits  to  break  through 
in  the  first  instance  to  the  number  of  fourteen,  no  less 
than  six  of  them  must  close  up  and  disappear  before  the 
stage  with  only  eig  a  gill-slits  on  each  side  of  the  body  is 
arrived  at.  The  six  slits  which  are  to  close  include  the 
first  and  the  five  posterior  primary  slits.  In  the  larva 
shown  in  Fig.  80,  the  tenth  and  eleventh  primary  slits 
would  have  to  close  at  a  later  stage  ;  the  twelfth  is  on  the 
point  of  closure,  and  its  walls  present  the  characteristic 
coarsely  granular  appearance  spoken  of  above,  while  the 
thirteenth  and  fourteenth  slits  have  entirely  vanished. 

In  addition  to  the  fact  of  the  closure  of  these  primary 
slitS;  it  is  important  also  lo  emphasise  the  fact  that  they 
disappear  without  leaving  a  trace  behind.  In  the  higher 
Vertebrates  there  are  a  number  of  structures  not  only  di- 
rectly connected  at  some  stage  of  development  with  the 
pharyngeal  wall,  but  also  at  some  distance  removed  from 
it,  which  various  morphologists  have  interpreted  as  the 
remnants  of  ancestral  gill-clefts,  without  sufficiently  con- 
sidering the  question  whether  gill-clefts  were  in  the  habit 
of  leaving  their  mark  behind  them.^  In  Amphioxus,  at 
all  events,  they  do  not. 


The  Adjustment  of  tJie  Mont/i,  etc. 

While  the  gill-slits  have  been  adjusting  themselves  to 
their  definitive  positions,  the  mouth  has  also  been  sub- 
jected to  a  peculiar  kind  of  growth,  which  results  in  its 
bending  round  the  front  end  of  the  pharyngeal  wall,  and 
ultimately  assuming  an  anterior  and  median  position,  as 
we  find  it  in  the  adult. 


(J^^ 


144 


DEVEI.OI'MEXT   OF  AMPHIOXUS. 


In  Fig.  81,  a  larva  corresponding  in  age  approximately 
to  that  of  Fig.  74  is  represented  as  seen  from  the  left  side. 

As  noted  above,  the  posterior  primary  slits  bend  nor- 
mally under  the  pharynx  at  this  stage,  and  some  of  them 
extend  as  much  on  one  side  of  the  body  as  on  the  other, 
being  continued  across  the  ventral  side  of  the  pharynx. 
The  great  feature  of  this  larva  is  the  relatively  prodigious 
mouth,  through  which  the  upper  portions  of  the  first  four 
primary  slits  can  be  seen. 

From  this  side  we  look  into  the  depths  of  the  prncoral 
pit,  having  only  seen  it  by  transparency  in  the  preceding 


M   At 


Fig.  81.  — Anterior  portion  of  larva,  with  thirteen  gill-iMts,  from  the  left  side. 
(After  Wll.i.KY.) 

o/f.  Olfactory  pit,  communicating  with  ncuropori".  .r.  "  Nephridium  "  of  Hat- 
scliek.  ti.t.  Sjiinal  cord.  ch.  Xotochord.  p.p.  Pr;voral  pit.  ex.  External  open- 
ing; of  club-sliapi'd  gland,  ci.  Rudiment  of  buccal  cirri,  p.b.  Peripharyngeal  band. 
til.  Mouth,    p.s^^.  Thirteenth  primary  slit. 


figures.  It  is  continued  backwards  into  a  ciliated  groove, 
which  abuts  on  the  dorsal  margin  of  the  mouth.  Prob- 
ably most  of  the  food  which  enters  the  mouth  passes 
along  this  groove. 

Below  the  pointed  anterior  extremity  of  the  mouth  is  to 
be  seen  the  external  aperture  of  the  club-shaped  gland, 
and  a  short  distance  behind  this  is  a  round,  refringent 
body,  which  has  become  differentiated  from  the  gelatinous 


/..IK I -.11.    /)!■: 1 1:1. ( )l'Ml:.\T. 


US 


connective  tissue  lying  below  the  epidermis,  and  repre- 
sents the  rudiment  of  the  first  element  of  the  cartilagi- 
nous skeleton  of  the  buccal  cirri. 

Running  parallel  with  the  lower  margin  of  the  mouth, 
and  curving  gently  upwards  to  the  dorsal  wall  of  the 
pharynx,  is  a  ciliated  band  proceeding  from  the  lower  limb 
of  the  endostyle,  and  corresponding  to  the  one  on  the  other 
side,  which  we  found  in  connexion  with  the  upper  portion 
of  the  endostyle.  Its  course  on  the  left  side  is  somewhat 
different  anteriorly  from  that  of  the  right  side,  owing  to 
the  position  and  size  of  the  mouth.     (Cf.  Figs.  78  and  81.) 

The  so-called  olfactory  pit,  which  arose  at  a  much  earlier 
stage  as  an  ectodermic  depression  above  and  in  connexion 
with  the  neuropore,  no  longer  lies  in  the  mid-dorsal  line  as 
in  Fig.  64,  but  it  has  been  displaced  to  the  left  .side  by  the 
upgrowth  of  the  dorsal  fin  (Fig.  81).  Here,  as  in  the  case 
of  the  anus,  the  development  of  a  median  fin  has  no  other 
effect  on  the  aperture  in  question  than  to  cause  it  to 
forsake  its  primitively  median  and  symmetrical  position 
and  to  assume  an  asymmetrical  position  on  the  left  side  of 
the  body.  This  is  important  to  bear  in  mind,  as  the  asym- 
metrical position  of  the  mouth  will  be  explained  below  on 
an  analogous  basis. 

For  the  present  it  is  sufficient  to  call  attention  to  the 
fact  that,  with  the  exception  of  the  gill-slits,  whose  primary 
unpaired  character  is  due  to  the  retarded  or  latent  develop- 
ment of  their  antimeres,  the  unpaired  openings  in  the 
body-wall  —  namely,  neuropore,  prceoral  pit,  external  aper- 
ture of  club-shaped  gland,  mouth,  and  anus  —  all  lie  on  the 
left  side  of  the  body. 

At  a  slightly  later  stage  than  the  preceding,  the  front 
end  of  the  mouth  is  found  to  be  no  longer  pointed,  but  to 
have  become  rounded  off,  and,  moreover,  to  lie  at  a  deeper 


II 


'W^ 


140 


/J/:  I  ■ELOPMKXT   Ol-    .  IMl'llli >XL  S. 


level  than  previously  (Fig.  82).  The  posterior  groove  of 
the  prxoral  pit  which  we  described  in  the  last  stage,  seems 
to  be  preparing  the  way  for  the  mouth  to  dip  inwards 
towards  the  right  wall  of  the  pharynx,  which,  in  fact,  it  has 
actually  begun  to  do. 

At  a  still  later  stage,  corresponding  to  that  shown  in 
Fig.  "JT,  the  shape  of  the  mouth  has  become  entirely  altered 
(Fig.  83) 

It  has  now  the  form  of  a  triangle  with  the  apex  directed 
backwards  and  the  base  standing  vertically  in  front.  \\\\\ 
the  ajicx  and  the  base  are  not  in  the  same  tangential  plane, 


m   /' 


Fig.  82.  —  Anterior  portion  of  larva  sonu'what  older  than  prutciling,  to  show 
comnu'nciny  adjustnu-nt  of  the  month.     (After  W'll.l.l-A'.) 

e,  Endostyle  seen  through  tlie  nionth.     Other  letters  as  above. 

the  former  being  on  the  left  side  of  the  body,  and  the  latter 
much  deeper  inwards;  in  fact,  just  below  the  skin  on  the 
right  side  of  the  body.     (Cf.  Fig.  77.) 

We  see,  therefore,  that  the  longitudinal  diameter  of  the 
larval  mouth  is  gradually  shortening.  It  is  eventually 
reduced  to  zero  when  the  right  and  left  sides  of  the  mouth 
or  velum  come  to  lie  opposite  to  one  another,  the  velum 
ultimately  attaining  a  circular  form  and  a  median  sub- 
vertical  position  underneath  the  oral  hood.  When  the 
larva  has  reached  the  age  to  which  F'-t.  i  i  refers,  the  right 


Vi\i\ 


/..  /A'  / './/.    Dl-.VEI.or.MEX  l\ 


«47 


half  of  the  velum  is  nearly  but  not  even  yet  quite  opposite 
to  the  left  half  (Fii;.  93). 

In  the  prccedinj;  sta<;e  (Fig.  'iz)  there  were  several 
additional  buccal  cartilages  added  to  the  first  one  which 
we  described.  In  the  present  stage  these  have  begun  to 
-row  outwards  so  as  to  produce  small  notches  in  the 
integument,  wiiicii  is  now  commencing  at  this  point  to 
torm  the  right  half  of  the  oral  hood.  The  left  half  of  the 
hitter  arises  as  a  downgrowth  of  the  integument  from  the 
upper  margin  of  the  pneoral  pit  and  its  posterior  continua- 
tion, the  above-mentioned  ciliateil  groove.  (Cf.  Figs.  81, 
S2,  and  83.)     The  hinder  portion  of  this  fold  is  at  first  on 


.i^tr'^f'''^**^^*  Hip^. 


F'g-  83.  —  Anterior  portion  of  still  older  liirva,  from  the  left  side,  to  show 
rh;ini;e  in  shape  and  position  of  the  mouth.     (After  W'll.l.KV.) 

J.ftters  as  above.  The  left  half  of  the  oral  hood  is  now  growing  down  over  the 
iiunith  and  prajoral  pit. 

a  level  with  the  dorsal  margin  of  the  mouth,  and  in  fact 
merges  into  the  latter,  but  subsequently  grows  over  it, 
extending  to  its  posterior  extremity,  where  it  meets  the 
ri-ht  half  of  the  oral  hood. 

It  is  obvious  from  the  above  description  and  figures  that 
a  large  part  of  the  right  wall  of  the  oral  hood  is  derived 
from  the  original  wall  of  the  snout  below  the  pra:oral  pit, 
and  so  an  explanation  is  afforded  of  the  fact  noted  in  the 
first  chapter  that  the  right  half  of  the  oral  hood  is  continu- 
i>u.s  round  the  anterior  extremity  of  the  notonhord  with 
the  cephalic  expansion  of  the  dorsal  fin.^ 


r-r 


In 'it; 


148 


DEVELOPMENT   OF  AM  PHI  OX  US. 


The  prneoral  pit  itself  is  absorbed,  as  it  were,  into  the 
oral  hood,  so  that  it  eventually  loses  its  independent  exist- 
ence as  a  pit,  although  the  sense-organ  of  the  pra^oral  pit 
persists  in  the  adult  as  a  deep  groove  in  the  dorsal  wall 
of  the  oral  hood  to  the  right  of  the  base  of  the  notochord. 
The  remaining  ciliated  epithelium  of  the  original  prreoral 
pit  increases  in  extent,,  and  grows  out  into  the  finger- 
shaped  tracts  which  we  have  already  described  as  being 
characteristic  of  the  inner  surface  of  the  oral  hood,  consti- 
tuting the  so-called  "  Raderogan."     {Cf.  Fig.  3.) 

Equalisation  of  the  Gill-slits. 

In  the  stage  next  succeeding  that  of  which  a  ventral 
view  is  given  in  Fig.  80,  the  first  eight  primary  slits  —  that 
is  to  say,  from  the  original  second  to  the  ninth  inclusive  — 


C5IBi"T7s;j ,:.,  i?;^'?!?",'^''-"*"* 


V     Pb 


m 


Fig.  84.  —  L;irv:i  t()\\;ird  iIr-  cIdsl-  of  the  metamorphosis,  from  the  left  siili. 
(After  \Vll.l.i:v.) 

o.  Olfactory  jiit.  v.  W'liim.  p.h.  lVripharynt:;e;il  band.  e.  Endostyle.  p.s-.  Second 
primary  slit,  the  first  liavinfr  closed  up.  w.  Left  metapleur.  j.u.  Floor  of  atrium. 
f.s^-.  /..r'^.  \'estiges  of  thf:  twelfth  ami  thirteenth  primary  slits. 


have  become  definitely  established  on  the  left  side,  their 
longitudinal  and  vertical  axes  are  equalised,  and  in  most 
of  them  the  tongue-bars  are  completely  formed  (Fig.  84). 
No  tongue-bar  is  formed  in  the  first  slit  on  either  side,  and 
this  slit  apparently  remains  as  a  rule  simple  throughout 
life. 


m 


LARVAL   J>/:  VEI.OPMEXT. 


149 


ii| 


In  Fig.  84  the  last  indications  of  the  twelfth  and  thir- 
teenth primary  slits  are  to  be  observed  as  slight  depres- 
sions in  the  floor  of  the  pharynx  in  the  mid-ventral  line. 
The  tenth  and  eleventh  slits  would  close  up  later. 

It  should  be  pointed  out  that  the  closure  of  the  poste- 
rior primary  slits  does  not  proceed  in  perfect  correspond- 
ence with  the  age  of  the  larva,  but  takes  place  sometimes 
at  an  earlier  and  sometimes  at  a  later  stage  than  here 
depicted. 

The  gill-slits  of  both  sides  now  begin  to  elongate  in 
the  vertical  direction  (Fig.  93),  and  eventually  a  very  well- 
marked  stage  is  reached,  which  is  characterised  by  the 
l)resence  of  eight  pairs  of  gill-clefts.  This  latter  stage 
would  appear  to  have  a  considerable  duration,  and,  as  it 
stands  on  the  borderland  between  the  larva  and  the  adult, 
and  forms  the  commencement  of  what  may  be  called  the 
adolcscoit  period  of  the  development,  it  may  well  be 
regarded  as  a  critical  stage.  By  this  time  the  young 
Amphioxus  has  given  up  its  free  pelagic  life  in  the  open 
sea,  and  has  commenced  to  burrow  in  the  sand,  which  it 
continues  to  do  for  the  rest  of  its  life.* 

Fitrt/icr  Growt/i  of  Endostyle,  etc. 

At  the  point  at  which  we  left  the  endostyle,  its  two 
halves  were  in  the  relation  to  one  another  of  upper  and 
lower.  During  the  steps  in  the  metamorphosis  which  we 
liave  recorded  above,  the  upper  half  of  the  endostyle  is 
brought  down  to  the  same  level  as  the  lower  half  on  the 
tight  side  of  it,  and  so  the  definite  form  of  the  endostyle 
is  established  by  the  conjunction  of  its  right  and  left 
halves.      It  then   proceeds  to  grow  backwards  along  the 

*  The  (luratit)n  of  the  larval  development  of  Amphioxus  may  be  estimated 
at  al)out  three  months. 


I 


/^ 


If  ' 


i'i 


llp^t 


150 


DEVELOPMENT   OE  AMrillOXUS. 


base  of  the  pharynx  between  the  two  rows  of  gilt-slits, 
but  does  not  reach  the  posterior  end  of  the  pharynx  until 
a  much  later  period.'" 

The  features  in  the  developmcii  of  the  endostyle  which 
ought  to  be  especially  emphasised  are,  firstly,  its  direc- 
tion of  growth  from  before  backwards,  and  secondly,  its 
primary  anterior  position  in  the  wall  of  the  pharynx  in 
front  of  all  the  gill-slits. 

In  connexion  with  the  modification  in  the  shape  and 
position  of  the  mouth,  as  described  above,  it  is  important 
to  insist  on  the  fact  that  the  mouth  of  the  larva  is  directly 
converted  into  the  velum  of  the  adult,  while  the  oral  hood 
which  grows  over  the  mouth  is  a  new  formation. 

During  the  period  of  the  metamorphosis  the  larva  does 
not  increase  in  length.  It  is  rather  a  readjustment  of 
parts  which  is  then  taking  place  than  an  increase  in  bulk 
which  is  the  symbol  of  active  growth.  From  the  time  of 
the  first  indication  of  the  secondary  slits  (Fig.  74)  till 
after  the  completion  of  the  passage  of  the  primary  slits 
from  the  right  to  the  left  side  of  the  body,  the  average 
length  of  the  larva  may  be  taken  as  approximately 
3.5  mm. 

The  adolescent  period  is  essentially  the  period  of  active 
growth  in  bulk  and  mrturity.  The  increase  in  length 
during  this  period  does  not,  however,  depend  on  the 
addition  of  new  myotomes  to  those  already  formed,  but 
merely  on  the  progressive  growth  in  size  of  the  latter. 
The  full  complement  ot  myotomes  was  developed  during 
the  early  larval  period,  and  is  present  in  the  larva  repre- 
sented in  Fig.  74. 


jI. 


LAK  \  'AL   DE I  'EL  0 PATENT. 


151 


Development  of  Reproductive  Organs. 

One  of  the  most  interesting  events  which  we  have  now 
to  chronicle  is  the  development  of  the  reproductive  organs. 
This  commences  when  the  young  Amphioxus  has  reached 
the  length  of  about  5  mm. 

Our  knowledge  of  the  details  of  the  processes  involved 
in  the  formation  of  the  genital  organs  is  again  due  to  the 
work  of   BovERi,  who  has  made  the  discovery   that  the 


ao 


met 


Fig.  85.  —  Transverse  section  through  the  pharyngeal  region  of  a  young 
iniliviihial  of  5  mm.,  to  show  place  of  origin  of  sexual  elements.     (After  H()\  I  Ki.) 

A  Fascia,  ex.  Portion  of  cojlom,  which  will  form  the  endostylar  cuelom. 
Hi,'.  Primitive  sexual  cells  in  the  lower  angle  of  the  niyoca'l.    Other  letters  as  above. 

primitive  sexual  cells  arise  in  the  cavity  of  the  myotome 
by  differentiation  of  certain  of  the  epithelial  cells  lining 
the  myocoel. 

It  had  previously  been  assumed  that  they  were  derivatives 


I  J.  > 


152 


DEVELOPMENT  OE  AMI'/flOXCS. 


of  the  peritoneal  epithelium  lining  the  general  body-cavity. 
The  fact  that  they  arise  in  the  way  shown  by  Boveri  is  one 
of  great  morphological  importance. 

In  a  transverse  section  of  a  young  individual  5  mm. 
in  length,  the  primitive  sexual  cells  are  to  be  recognised 
as  a  closely  packed  group  of  cells,  with  large  nuclei  in  the 
lower  angle  of  the  myotome  ;  that  is,  in  the  angle  formed 
by  the  membrane  which  divides  the  myoccel  from  the 
sjilanchnoccel,  which  we  may  call  the  intcrcoelic  monhranc, 
with  the  cutis  (Fig.  85).  Since  the  myotomes  of  one  side 
alternate  with  those  of  the  other,  so  do  the  centres  of 


ml 
If-  ■ 


'4. 


\.\:  m 


Fig.  86.  —  Longitudinal  views  of  the  developing  gonads,  obtained  by  dissecting 
out  the  ventral  borders  of  the  myotomes.     {After  RoVK.Rl.) 

u.^i,'.  Primitive  sexual  cells  arising  from  the  myocuelic  epithelium;  the  nuclei 
scattered  about  the  surface  of  the  preparations  also  belong  to  the  myocuelic 
epithelium. 

formation  of  the  primitive  sexual  cells,  and  in  a  given 
section,  as  in  Fig.  85,  only  one  such  centre  is  to  be  observed 
on  the  right  or  left  side  of  the  section,  as  the  case  may  be. 
Its  actual  position  in  the  longitudinal  aspect  of  the  myo- 
tome is  shown  in  Fig.  86  A,  B,  and  C.  The  formative 
centres  of  the  primitive  sexual  cells  lie  at  first  in  the  angle 
mentioned  above,  but  applied  to  the  posterior  faces  of  the 
dissepiments  between  the  myotomes  (Fig.  86  A). 

At  a  somewhat  later  stage,  having  slightly  increased  in 
bulk,  they  begin  to  push  the  dissepiments  before  them 


L.IK  I  AL   J)/'.  1 7:7. 0/\]//:X7\ 


153 


SO  as  to  make  a  projection  into  the  myocoel  in  front  (Fij;. 

86  7),  C).     This  projection  of  the  primitive  gonad  into  the 

myocoel  next  in  front  of  that  to  which  it  originally  belonged, 

is    gradually    carried    to    such    an 

extent    that    the   gonad    becomes 

entirely  shut  off  from  its  original 

myocoel  and  hangs  freely  into  the 

next   one,    being   connected    by    a 

short  stalk  with  the  anterior  face       Fig.  87.  —  similar  prepam- 

.  ,  ,  tioii  as  the  prccciiing,  sliowin  • 

of  the  dlSSepmient  and  surrounded    a  lat.-r  stage  in  the  .levelopnunt 

by  a  membrane  which  is  obviously  "/  ""'  i'""""^'-*  ^onad.  (Atter 
derived  from,  and  for  some  time 

continuous  with,  the  original  dissepiment  (Fig.  87).  In 
correlation  with  the  increase  in  size  of  the  primitive  gonad, 
an  evagination  of  the  basal  wall  of  the  myocoel  in  which  it 
now  lies,  takes  place,  and  by  the  time  the  young  Amphi- 


Fig.  88.  —  Preparation   showing  the   rliomboicial    pouches    of    the    niyt)cii.'l 
wiiicii  project  into  the  atrial  cavity.     (.Mter  Bdvkki.) 
I'his  condition  is  found  in  individuals  of  13-14  nun. 


oxus  has  attained  a  length  of  13  or  14  mm.  there  is,  in 
connexion  with  each  primitive  gonad,  a  wide  rhomboidal 
expansion  of  the  lower  portion  of  each  corresponding 
myocoel  projecting  into  the  atrial  cavity  (Fig.  88). 

The  cavity  of  these  sacs,  to  the  wall  of  which  the  gonads 
are  at  this  stage  still  united  by  a  stalk,  cons;  itutes  the  so- 
called  pcrigiviadial  ca:lom}^  or  cavity  of  the  gonadic 
pouches,  which,  at  the  time  of  sexual  maturity,  is  entiicly 
filled  up  by  the  sexual  elements. 


154 


DEVELOPMENT   OE  AM  PHI  OKI'S. 


m 


I  ' 


The  gonadic  pouches  next  become  gradually  constricted 
off  from  the  myocoelic  spaces,  and  eventually  lose  all  com 
munication  with  them.     In  the  midst  of  the  at  first  solid 

mass  of  primitive  sexual 
cells  a  cavity  subsequently 
appears,  and  the  gonad  be- 
comes a  hollow  sac  (Fig.  89). 
In  the  course  of  its  fur- 
ther growth  the  gonadic  sac 
(not  to  be  confused  with  the 
gonadic  pouch  in  which  it 
lies)  grows  out  into  a  num- 

Fig.  89.  —  Portion  of  transverse  sec-  ,            f    1            i-              1             \ 

tion   througli  an   inilividual   of   13   mm.,  ^^^   ^^     lappCtS,    and    SO     DC- 

to  explain  the  conditions  observed  iu  comCS  a  raCCmOSC  rcproduC- 
precedint;  prei)aration.     (.After  B()\i',Rl.) 

fi.v.  iMood-vessei.  .^r,,.  Gonadic  sac.  tivc  gland  (Langcrhans). 

f.ir.c.     I'ericfonadial     coeloni      (eonadic  •t'i  •      ■^-  1        11 

pouclO.     /.«/.  Transverse  muscles.     The  The  prmiltlVC  SCXUal  Cclls 

index  line  to  xvbich  there  is  no  letter  remain    for    a    Considerable 

indicates  the  told  by  which  the  gonadic 

liouch  iiecomes  constricted  off  from  the    length    of    time    in    an    absO- 

'"-°'^°^'  lutely  indifferent  condition, 

and  it  is  impossible  to  distinguish  the  male  from  the 
female. 

According  to  Laxgekhaxs,  sexual  differentiation  does 
not  begin  to  take  place  until  the  individuals  have  reached 
a  length  of  17  mm.,  and  sometimes  it  does  not  occur  until 
a  much  later  period.  It  is  inaugurated  by  the  commence- 
ment of  the  processes  of  spermatogenesis  and  o\'ogenesis. 
There  are  no  accessory  sexual  characters  in  Amphioxus, 
and  the  sex  can  only  be  determined  by  an  examination  of 
the  reproductive  glands. 

The  segmental  arrangement  of  the  formative  centres 
of  the  reproductive  organs  at  the  base  of  the  myotomes 
is  again  met  with  in  the  embryonic  development  of  the 
Selachians,  as  shown  by  Ruckert  (Fig.  90).     Here,  also, 


•■  I  h 


m 

hi '  i 


LARVAL   L^El'ELOl'MEXT. 


155 


the  primitive  sexual  cells 
make  their  first  appearance 
in  the  segmentec'.  area  of 
the  trunk  at  the  base  of 
the  somites.  Later  on,  by 
differential  growth,  they 
come  to  lie  on  the  dorsal 
wall  of  the  unsegmented 
peritoneal  cavity,  and  their 
primitive  segmental  origin 
is  entirely  obscured  ;  while 
in  Amphioxus  the  primitive 
segmentation  of  the  gonads 
is  maintained  throughout 
life. 

This  forms  another  most 
interesting  example  of  the 
way  in  which  the  adult 
Amphioxus,  in  the  details 
of  its  organisation,  essen- 
tiallv  resembles  the  em- 
hryos  of  the  higher  types. 


Fig.  90.  —  Horizontal  section  tlirnugh 
the  ventral  portion  of  si.x  consecutive 
mesociermic  somites  of  an  embryo  of 
Pristiurus,  to  show  the  segmental  origin 
of  the  sexual  elements.   (After  RfCKl'.K  I ). 

I.  Cavities  of  somites.  g.c.  Sexual 
cells. 

This  observation  of  Riickert's  lias 
recently  i)een  doubted,  with  how  much 
justice  it  is  ditticult  to  say,  by  Misur 
(C,i\i;cn  das  Coito/oni.  Anat.  Anz.  I\. 
1894.     pp.  210-213). 


GENERAL   COXSIDKRATIONS. 

We  will  now  pass  on  to  give  a  general  interpretation  of 
some  of  the  principal  phenomena  which  are  presented  10 
us  in  the  development  of  Amphioxus. 

Larval  AsyiiiDirfjy. 

By  far  the  most  prominent  feature  of  the  fully  formed 
larva  is  its  astounding  asymmetry,  and  it  is  extremely 
important,  from  a  morphological  point  of  view,  to  form  a 
just  conception  of  it. 


"wmw 


156 


DEVELOPMENT  OF  AMPinOXUS. 


li: 


It 


The  phenomenon  of  asymmetry  manifests  itself  in  the 
larva  of  Amphioxus  under  several  very  different  aspects, 
and  is  occasioned  by  various  causes.  For  convenience  we 
may  classify  the  forms  of  asymmetry  which  we  have  to 
consider  under  three  main  divisions,  according  to  the  type 
of  organs  involved. 

1.  Median  Asymvictry.  — This  relates  to  such  structures 
as  lie  normally  in  the  middle  line,  whether  dorsal  or  ven- 
tral, but  which  have  been  mechanically  or  correlatively  dis- 
placed from  their  primitive  position  by  the  differential 
growth  of  neighbouring  parts.  Such  are  the  olfactory  pit 
and  neuropore,  the  anus,  the  mouth,  and  the  endostyle.  All 
these  are  essentially  and  primordially  median  and  unpaired 
structures.  We  have  already  dealt  with  the  neuropore 
and  anus,  while  the  mouth  and  endostyle  will  be  con- 
sidered below. 

2.  Bilateral  Asymmetry.  — This  refers  to  the  alternation 
of  paired  structures,  such  as  myotomes,  spinal  nerves,  gill- 
slits,  and  gonads,  which  we  have  already  noted  in  the  adult 
organisation.  Primarily  the  organ  of  one  side  lies  opposite 
to  its  antimere  of  the  other  side.  By  a  secondary  displace- 
ment it  comes  to  alternate  with  it.* 

3.  Unilateral  Asymmetry.  —  Next  to  the  asymmetrical 
mouth,  this  is  perhaps  the  most  striking  form  of  asym- 
metry which  the  larva  of  Amphioxus  exhibits.  It  relates 
to  those  structiires  which  belong  to  the  category  of  paired 
organs,  but  which,  in  the  course  of  the  larval  development, 
appear  unpaired  on  one  side  of  the  body.     Such  are  the 


*  When  K\\i.  myoca-lomic  pouches  first  appear  in  the  embryo  they  are 
placed  symmetricany.  At  an  early  stage,  however  (see  Fig.  63  5),  the  alter- 
nation sets  in.  This  involves  such  later-appearing  structures  as  the  spinal 
nerves  ami  gonads,  so  that  they  alternate  from  the  time  of  their  first  origin. 
The  alternation  of  the  gill-slits  would  seem  to  be  independent  of  that  of  the 
myotomes. 


LAR I W  I.    DE I  'EL  OrMEA'T. 


>57 


gill-slits  and  the  prneoral  pit.  As  described  in  the  fore- 
going pages  the  asymmetry  of  the  pra^oral  pit  is  a  second- 
ary occurrence,  since  it  arises  at  first  as  one  of  a  pair  of 
symmetrically  disposed  head-cavities,  or  anterior  intestinal 
diverticula,  while  the  unilateral  asymmetry  of  the  gill-slits 
is  ontogenetically  primary.  The  unilateral  gonads  of  tho 
species  of  Amphioxus  from  the  Bahamas  and  Torres 
Straits  also  belong  to  this  category. 

Although,  on  account  of  their  essentially  azygous  nature, 
the  mouth  and  endostyle  have  been  separated  from  the 
gill-slits  in  the  above  classification,  it  is  obvious  that  their 
asymmetrical  position  in  the  larva  must  be  ascribed  to  one 
and  the  same  cause.  In  the  succeeding  pages  we  shall 
endeavour  to  demonstrate  what  this  cause  was. 

Explanation  of  Asymmctty  of  Mouth  and  Gil/slits. 

It  is  quite  evident  that  the  primary  gill-slits  which 
appear  on  the  right  side  of  the  larva  belong  primitively, 
or  ancestrally,  to  the  left  side,  to  which,  in  fact,  they  are 
eventually  transferred.  Meanwhile,  the  left  side  of  the 
larval  pharyngeal  region  is  largely  occupied  by  the  huge 
oral  aperture. 

We  may  figure  to  ourselves  the  primitively  left-side  gill- 
slits  being  carried  over  to  the  right  side  by  a  semi-rotation 
from  left  to  right  of  the  pharyngeal  wall.  The  primitive 
right  side  of  the  pharynx  would  thus  be  crowded  out,  so  to 
sj)eak,  and  the  right-side  gill-slits  would  be  temporarily 
obliterated  owing  to  lack  of  room,  while  the  original  mid- 
ventral  line  would  be  carried  high  up  on  the  right  side, 
where,  in  point  of  fact,  it  is  plainly  indicated  by  the  bran- 
chial artery,  which  lies  actually  above  the  primary  gill-slits 
in  the  larva  (Fig.  74,  etc.). 


/y 


i$S 


Dl-.VEI.OI'MEXr   OF  AMP///0.\'US. 


Thus  the  actual  topot;raphical  comlitions  in  the  larva  do 
not  by  any  means  coincide  with  the  niorpholoj^ical  rela- 
tions of  parts,  since  the  morpholof^ical  miil-ventral  line  of 
the  pharynx  lies  hiy;h  up  on  the  riy;ht  side  of  the  body.  It 
sliould  be  carefully  noted  that  the  form  of  asymmetry 
which  we  are  now  considering;  only  affects  the  anterior 
portion  of  the  larval  body. 

The  same  semi-rotation  of  the  pharyngeal  region  which 
converted  the  primitive  left  side  of  the  larva  into  the 
actual  right  side  caused  the  primitively  median  mouth  to 
take  up  its  position  on  the  actual  left  side,  l^ut  since,  as 
we  have  noteil,  the  rotation  occurred  from  left  to  right, 
the  mouth  must  have  been  originally  situated  in  the 
median  dorsal  line. 

In  postulating  a  virtual  semi-rotation  of  the  ancestral 
pharyn.x,  we  do  not,  of  course,  mean  to  suggest  the  prob- 
ability of  an  actual  movement  in  bulk  about  the  longi- 
tudinal a.xis,  but  merely  that  the  formative  centra's  of  the 
various  structures  belonging  to  this  region  of  the  body 
(gill-slits,  mouth,  endostyle,  etc.)  have,  by  the  correlated 
interaction  of  their  component  cell-groups,  been  diverted 
from  their  ancestral  relations  through  the  intercalation,  in 
the  course  of  the  progressive  evolution  of  the  organism,  of 
a  new  and  disturbing  element. 

We  are  now  in  a  position  to  say  what  this  disturbing 
element  is.  It  is  the  secondary  forward  extension  of  the 
notochord  beyond  the  limits  oi  the  dorsal  nerve-tube  to 
the  tip  of  the  snout.  As  already  stated,  there  is  direct 
evidence  to  show  that  this  is  a  secondary  and  not  an  an- 
cestral feature,  inasmuch  as  in  the  young  embryo  (Fig. 
6S  bis)  the  notochord  is  removed  fro.n  the  anterior  extrem- 
ity of  the  body  by  a  very  appreciable  interval,  which  is  oc- 
cupied by  that  portion  of  the  archenteron  which  gives  rise 


LARVAL   DIAL:LOrME\T. 


"59 


to  the  head-cavities.  Moreover,  as  was  pointed  out  abo\e, 
the  iluisul  groove  of  tiie  archenteron,  which  gives  rise  to 
the  iidiochord,  remains  open  into  the  archenteric  cavitv 
in  the  region  of  the  first  myotome,  and  even  somewhat 
behind  tlie  le\'el  of  the  neuropore,  for  some  time  after 
its  walls  have  ajjproximated  to  form  the  solid  notochord 
behind  this  region. 

The  forward  extension  of  the  notochord  in  Amphioxus 
is,  thvretore,  dc  facto,  to  a  large  extent  an  ontogenetic 
phenomenon,  although,  from  the  very  beginning,  it  shows 
what  may  be  described  as  a  precocious  teiulency  to  extend 
beyond  the  nerve-tube.  We  shall  also  fintl  that  theie  is 
every  reason  to  sujipose  that  it  is  a  cenogenetic,  and  not  a 
palingenetic,  feature.'- 

Since  we  know  for  an  actual  fact  that  the  primary  gill- 
slits  of  the  larva  belong  ancestrally  to  the  left  side,  it  fol- 
lows as  an  absolute  topographical  necessity  that  the  mouth 
has  been  brought  to  one  side  from  an  originally  median 
dorsal  position,  X-)-^'  \.\\c  same  semi-rotation  of  the  pliai-ynx 
(in  the  sense  explained  above)  wdiich  has  demonstrably 
carried  the  primitive  left-side  gill-slits  under  the  pharynx 
up  to  the  right  side  of  the  larva.  But  this  is  not  the  only 
criterion  by  which  we  can  judge  of  the  ancestral  position 
of  the  mouth. 

In  the  larvae  of  the  Ascidians,  the  nearest  existing  rel- 

•al  lobe  and  a  neuro- 


iphi 


pr; 


pore,  wl 


hich  opens  at  first  to  the  exterior  in  the  mid-dorsal 
line,  just  as  in  Amphioxus.  l^ut  in  contrast  to  the  latter 
form  the  notochord  does  not  extend  forwards  into  the  re- 
git)n  of  the  jira^oral  lobe,  but  it  stops  short  behind  the 
cerebral  vesicle. 

Immediately  in  front  of  the  neuropore,  in  the  Ascidian 
larva,  the  wall  of  the  pharynx  comes  into  contact  with  tin- 


m 


i6o 


/)/:n:/.(>/'.\//:X7-  oi-  ami'iiioxus. 


ectoderm  and  fuses  with  it,  and  then  at  the  point  of  fusion 
a  perforation  takes  place,  and  the  mouth  is  established  in 
the  mid-ilorsal  line.  During  the  formation  of  the  mouth 
the  neuropore  temporarily  closes  up,  but  subsequently  it 
leopens  —  i)ito  the  mouth. 

In  Amphioxus  we  can  only  assume  that  in  cori elation 
with  the  forward  extension  of  the  notochord,  the  mouth 
was  compelled  to  forsake  its  primitive  relations  to  the 
neuropore  and  to  move  to  one  side  so  as  to  make  way  for 
the  notochord.  The  j^rowth  of  the  latter  to  the  front  end 
of  the  body  obviously  prevents  the  wall  of  the  j)harynx 
from  coming  into  contact  with  the  ectoderm  in  the  mid- 
dorsal  line,  while  it  leaves  the  neuropore  unaffected,  since 
the  nerve-tube  is  essentially  dorsal  to  the  notochord,  and 
the  pharynx,  on  the  other  hand,  essentially  ventral  to  it. 

This  explains  the  fact  that  the  hypophysis  (olfactory  pit) 
of  Amphioxus  opens  dorsally  directly  to  the  exterior  instead 
of  into  the  mouth  as  it  does  in  the  Ascidian. 

The  secondary  gill-slits  —  that  is,  those  belonging  to  the 
primitive  right  side  of  the  body  —  present  an  interesting 
instance  of  retarded  or  latent  development.  This  is  due 
to  the  fact  that  their  own  side  of  the  body  is  at  first 
usurped  by  their  primitive  antimeres,  the  so-called  primary 
slits,  as  a  result  of  which  they  have  themselves  been 
temporarily  crowded  out  as  mentioned  above.  In  con- 
sequence of  their  retardation,  when  they  do  appear  to 
inaugurate  the  process  of  symmetrisation,  they  do  not 
conform  to  the  method  in  which  metameric  structures  are 
normally  produced,  but  most  of  them  —  namely,  from  the 
second  to  the  seventh  inclusive  —  arise  simultaneously 
while  the  first  and  the  eighth  arise  somewhat  later. 


I.- 


I.AKVM    PF.VEl.OrMEXT. 


xCn 


Larval  Asymmetry  not  Adaptive  and  not  Advantas^eoux  ; 
Forward  Jixtension  of  Noto'liord  Adaptive  and  ^Idvan- 
taj^eons. 

The  conclusion  to  bo  drawn  from  the  above  considera- 
tions is  that  the  remarkable  asymmetry  of  the  larva  of 
i'\mphioxus,  in  respect  of  the  pharynx  and  the  parts  con- 
nected with  it,  is  of  no  s])ecific  advanta<;e  whatever  to  the 
larva,  but  is  merely  a  stage,  wliich  has  been  preserved  in 
the  ontogeny,  of  a  topographical  readjustment  of  j)arts 
necessitated  by  the  removal  of  the  mouth  from  its  primi- 
tive mid-dorsal  position  in  consequence  of  the  secondary 
iorward  extension  of  the  notochord,  which  has  thus  caused 
a  virtual  semi-rotation  of  the  pharyngeal  region  of  the 
botly.  On  the  other  hand,  the  forward  extension  of  the 
notochord  is  a  distinct  advantage  in  later  life,  since,  by 
giving  resistancy  to  the  snout,  it  enables  the  animal  to 
burrow  its  way  into  the  sand  with  such  astonishing  facility, 
while  the  fact  that  it  grows  to  the  front  end  of  the  body  at 
a  very  early  stage  in  the  embryonic  develojiment,  long 
before  it  comes  to  be  }nit  to  this  definite  use,  must  be 
regarded  as  an  instance  of /r^wr/Vv/jr  development  of  which 
there  are  numerous  and  otherwise  inexplicable  examj^les 
in  the  field  of  comparative  emoryology. 

The  larval  asymmetry  of  Amphioxus  is  therefore  a  purely 
secondary  or  cenogenetic  feature,  and  has  no  directly  ances- 
tral or  palingenetic  significance,  although,  as  we  have  shown 
above,  it  serves  indirectly  as  a  clue  to  what  the  ancestral 
condition  was.  At  the  same  time  it  is  a  primary  feature 
in  the  actual  ontogeny  ;  that  is  to  say,  the  asymmetrical 
structures  (mouth  and  gill-slits)  arise  in  situ,  and  are  not 
removed  in  the  individual  development   from   a  primary 


fJb 


"^IP 


1 6: 


DEVEI.OPMKXT   OJ-   .LyPI/IOXrS. 


symmetrical  to  a  secondary  asymmetrical  position,  as  is  the 
case,  for  instance,  with  the  neuropore. 

It  may  appear  paradoxical,  but  is  nevertheless  correct,  to 
say  that  in  the  ontogeny  the  mouth  and  gill-slits  appear 
pyiniarily  in  a  secondary  position. 

It  is  quite  evident  that  the  asymmetry  of  the  larva  of 
Amphioxus  is  of  a  totally  different  character  to  the  well- 
known  asymmetry  of  the  flat-fishes  or  Plciironirtidcc 
(turbot,  sole,  plaice,  halibut,  flounder,  etc.).  The  latter 
are  hatched  as  perfectly  symmetrical  larvas  with  eyes  quite 
opposite  to  one  another.  Then,  in  adaptation  to  a  life  at 
the  bottom  of  the  sea,  after  a  short  pelagic  existence  they 
turn  over  on  one  side,  in  some  species  the  right  side,  and 
in  others  the  left,  and  the  eye  of  that  side  moves  over  the 
snout,  sometimes  even  through  the  snout,  to  the  other 
side,  and  so  the  eyes  come  to  lie  on  the  same  side.  In  this 
case,  therefore,  the  asymmetry,  which  is  secondary  in  every 
sense  of  the  word,  is  the  result  of  a  special  adaptation  to  a 
particular  habit  of  life,  and  is  accordingly  of  the  greatest 
advantage  to  the  fishes  which  possess  it. 

On  the  other  hand,  its  extraordinary  asymmetry  is  of 
no  conceivable  advantage  to  the  larva  of  Amphioxus,  and 
does  not  represent  an  adaptation  to  any  peculiar  mode  of 
existence  whatever. 

It  is  rather  the  mechanical,  incidental,  accessory,  and 
subsidiary  accompaniment  of  another  organic  change  which 
is  both  advantageous  and  adaptive,  namely,  the  forward 
extension  of  the  notochord  ;  and  while  the  excessive  asym- 
metry is  intlifferent  to  the  pelagic  larva,  it  would  be  posi- 
tively detrimenla   to  the  adult. 

Thus  ii,  all  respects  the  larval  asymmetry  of  Amphioxus 
is  the  precise  converse  of  the  adult  asymmetry  of  the 
Pleuronectida;.^'^ 


AMrinoXiS  A\D  AM.yoCiEJES. 


163 


AMPHIOXUS   AND   AMMOOITES. 

We  will  now  pass  on  to  consider  what  new  light  the 
larval  development  of  Amphioxus  throws  on  its  relation- 
ship to  the  craniate  Vertebrates. 

As  a  type  of  the  latter  with  which  to  make  the  com- 
parison, we  will  select  Ai)nnoca:tcs,  the  larva  of  the  lamprey, 
Petromyzon,  which  is  the  nearest  relative  of  Amphioxus 
amonff  the  Craniota. 


•ard 


Ncrviis  Branchialis  Vagi. 

Although  Amniocoetes  possesses  an  organisation  which, 
especially  in  virtue  of  its  nervous  system  and  sense- 
organs,  entitles  it  to  an  undoubted  place  among  the 
craniate  Vertebrates,  yet,  on  the  v/hole  its  structural  ele- 
ments remain  in  such  a  relatively  simple  condition  of 
elaboration  that  it  readily  adapts  itself  to  a  comparison 
with  Amphioxus. 

At  the  same  time  the  system  of  ganglia  and  peripheral 
cranial  nerves  indicated  in  Fig.  91  will  show  what  a  great 
gap  there  is  between  the  two  forms.  Nevertheless,  a 
nerve  corresponding  to  that  which  lies  over  the  gill-slits 
in  Fig.  91,  the  ncrvus  branchialis  vagi,  has  recently  been 
discovered  in  Amphioxus  by  van  Wijhe,  so  that  there 
need  be  no  difficulty  in  comparing  the  pharyngeal  tract 
of  Ammocoetes  with  that  of  Amphioxus. 

It  may  be  added  here  that  the  nerve-supply  of  the 
pharynx  of  Amphioxus  was  described  as  a  branchial  plexus 
by  RoHON  and  Fusahi,  but  the  origin  of  the  nerves  which 
gave  rise  to  the  plexus  was  not  satisfactorily  determined, 
beyond  the  fact  that  they  arose  from  the  rami  viscerales 
of  the  dorsal  spinal   nerves.      Van*  Wijiie   also  was   not 


vfim  iH '  iipftP'i 


164 


DEVELOIWEXT   OF  AMPIirOXl'S. 


able  to  determine  the  precise  origin  of  the  longitudinal 
nerve  discovered  by  him.  This  nerve,  which  lies  on  either 
side  at  the  place  where  the  ligamentum  denticulatum 
passes  into  the  gelatinous  lamella  derived  from  the  inter- 
coelic  membrane,  gives  off  the  branches  which  form  the 
"branchial  plexus."  Van  Wijhe  states  that  the  origin  of 
the  "ramus  branchialis  vagi"  of  Amphioxus  is  to  be 
sought   in   the    eighth  to  the  tenth  dorsal  spinal  nerves. 


» 


/     /irn  sf         Jiy 


Fig.  91.  —  Anterior  portion  of  young  Ammoca'tes  ol  4  nun.,  to  show  extension 
of  brain,  origin  of  cndostyle  (thyroid),  relations  of  bianchial  nerves,  etc.     (Aflei 

KUl'l'I-IK.) 

/,  //,  ///,  /  y.  The  so-c;illi'il  "  I  Iauptg;i.nglia."  /  ami  //.  'I'rigeniinus. 
///.  Acustico-faciah--      /  / '.  (iiossopharyngeus.      / '.  Vagus. 

an.  Aiuhtory  capsule,  c/i.  Xotochortl.  t.  Enciostyle  (hypoljranchial  groove, 
thyroid),  hy.  Hypo[ihysis,  in  front  of  wiiich  is  the  nasal  groove.  //./.  Nervus 
laterahs.  n.br.  Nervus  brancliialis.  o.p.  live.  /.  Pineal  body  (epiphvsis). 
p.m.  I'ru-'oral  endodermic  jujuch  (median  portion  of  privniandihular  cavity. 
jA  St<3mod(X'nm.  /,  /'///.  I'"irst  and  eighth  gill-pouches;  the  small  circles  Ijehiiid 
the  gill-pouches  indicate  the  iiositioi.'S  of  the  external  openings  of  the  gill-pouches, 
which  will  become  perforated  later.  The  small  black  sjjots  in  front  of  tlie  (later 
appearing)  external  openings  represent  the  so-called  ^i^^aiii;/iLi  piucti  ematica. 

He  found  that  the  nerve  curved  ventrahvards  in  front  and 
passed  downwartls  through  the  intercoelic  membrane  until 
it  reached  the  level  of  the  ventral  transverse  muscles  in 
front  of  the  visceral  braich  of  the  eleventii  spinal  nerve. 
He  was  unable  to  follow  it  further  in  the  complex  ner\x 
plexus  which  lies  on  the  surface  of  the  muscles.  It  is 
probable,  however,  that   the  branchial   nerve  arises  from 


AMrniOXUS  AXD   .IMMOCfh  TKS. 


165 


ity. 
ind 
10s, 
;itor 


the  visceral  branch  of  the  eighth,  ninth,  or  tenth  spinal 
nerve.* 

StoviodccHui,  Hypophysis,  and  Gill-slits. 

It  is  a  common  fact  that  the  time  and  order  of  forma- 
tion of  corresponding  parts  differ  greatly  in  the  dexclop- 
ment  of  different  species.  Thus  in  Ammocoetes,  at  the 
stage  shown  in  T'ig.  91,  the  definitive  mouth,  C(MTcspond- 
ing  to  the  velum  in  Am])hioxus,  has  not  yet  formed,  but 
the  equivalent  of  the  oral  hood  is  already  i)resent  in  the 
form  of  a  deep  in-pushing  of  the  ectoderm  which,  at  its 
blind  end,  is  closely  applied  to  the  anterior  endodermic 
wall.  The  mouth  will  break  through  later  in  the  middle 
of  the  area  of  contact  between  ectoderm  and  endoderm. 

This  ectodermic  invagination,  whose  cavity  is  probably 
the  honologue  of  the  vestibule  formed  by  the  oral  hood 
which  leads  into  the  mouth  in  Amphio.xus,  is  known  as 
tl'iC  stoiuuhvHui.  Immediately  in  front  of  the  stomod(eum 
is  another  ectodermic  involution  which  is  in  contact  with 
the  front  of  the  brain,  and  is  known  as  the  hypophysis  ox 
pituitary  bodyM 

It  will  appear  later  that  this  is  the  probable  equivalent 
of  the  so-called  olfactory  i)it  of  Amphioxus. 

In  the  wall  of  the  pharynx  of  Ammocoetes  there  are,  at 
this  stage,  the  indications  of  eight  pairs  of  gill-slits.  The\' 
have  not  yet,  however,  broken  through  to  the  exterior,  but 
consist  of  a  succession  of  hollow  outgrowths  of  the  phar- 
ynx stretching  towards  the  ectoderm  with  which  they  will 
eventually  fuse  (Fig.  92  A,  B,  C). 

In  the  case,  however,  of  the  first  pair  of  gill-pouches, 


Dm 


*  It  is  not  iniiio.isililc  that  many  of  tlu-  lanii  viscerales  may  send  u[>  hraiiclics 
ti)  the  l)ranchial  plexus,  as  was  indeed  descriljed  liy  Rohon.  In  tliis  case, 
Van  Wijhc's  nerve  would  he  of  the  nature  of  a  collector. 


1 66 


DEVELOPMENT   OF  AMPII/OXUS. 


Wkmm. 


it  does  not  come  to  a  fusion  with  the  ectoderm  ;  but  in- 
stead they  begin  to  undergo  a  retrogressive  development 
and  eventually  flatten  completely  out  (Fig.  92  B).  They 
are  thus  shown  to  be  rudimentary  structures,  morphologi- 
cally representing  the  first  pair  of  gill-clefts,  but  never 
achieving  their  full  development. 


Fig.  92.  —  Horizontal  sections  through  the  pharyngeal  region  of  Ammocoetcs, 
to  show  the  relation  of  the  first  pair  of  gill-pouches  to  the  perijjharyngeai  grouV?s. 
(After  UoiiKN.) 

A.  Two  clays  after  hatching ;   first  pair  of  gill-pouches  well  developed. 

B.  Six  days  after  hatching ;  first  pair  of  gill-pouches  flattened  out. 

C.  Nine  days  after  hatching;  appearance  of  peripharyngeal  grooves. 
I-Vlll.     Gill-pouches,     b.v).     Body-wall.     oes.     (I'"sophagus.     ///.     Pharynx. 

ph.g.  Periphai-yngeal  groove,    st.  Stoinodocum.     vel.  Velum. 


As  to  their  position,  they  occupy  the  extreme  anterior 
angles  of  the  pharynx  formed  by  its  lateral  walls  with  the 
anterior  transverse  wall  against  which  the  stomodceum  is 
applied.  Whatever  may  be  the  reason  for  it,  the  atrophy 
of  the  first  pair  of  gill-pouches  in  Amnioctetes  is  of  pre- 
cisely the  same  natiuv  as  the  atrophy  of  the  first  gill-slit 
in  Amphioxus,  with  the  tlistinction  that  the  latter  actually 
opens  to  the  exterior  for  a  lime. 


i^ 


AM  PHI  ox  us  Axn  ammocoltes.. 


167 


Endostylc  or  Hypobraiic/iial  Groove. 

At  a  stage  in  the  development  of  Ammocoetes  which 
precedes  the  flattening  out  of  the  anterior  gill-pouches, 
a  median  depression  occurs  in  the  extreme  anterior 
region  of  the  ventral  wall  of  the  jjharynx  hetween  the 
first  pair  of  gill-pouches.  In  its  production  the  wall  of 
the  pharynx  at  this  region  projects  itself  ventrally  and 
slightly  forward.  This  groove,  which  is  known  as  the 
Itypobranchial ^^roovc ,  develops  in  the  direction  from  hefore 
backwards,  and  eventually  extends  backwards  as  a  longi- 
tudinal groove  as  far  as  the  fifth  pair  of  gill-pouches 
(Fig.  91). 

WiLHELM  MuLLER  showcd  that  it  was  the  homologue 
of  the  endostylc  of  Ascidians  and  Amphioxus,  antl  he  has 
been  amply  confirmed  by  Dohrx.  It  agrees  with  the  lat- 
ter structure  in  its  origin  at  the  anterior  extremity  of  the 
pharynx  and  subsecjuent  growth  backwards  and  in  its 
histological  structure,  the  most  marked  feature  of  the  lat- 
ter being  the  four  longitudinal  rows  of  gland-cells  which 
were  noted  above  in  the  endostylc  of  Amphioxus.  (Cf.  Fig. 
13,)     Like  the  latter,  also,  it  is  a  slime-secreting  gland. 

In  Ammocoetes  the  hypobranchial  groove  becomes 
largely  shut  off  from  the  cavity  of  the  jiharynx  by  the 
gradual  ingrowth  of  a  diai)hragm-like  lamella  which  pro- 
ceeds from  the  angle  made  by  the  grcjove  in  front  with  the 
anterior  wail  of  the  pharynx  (Fig.  91).  Subsequently  a 
similar  diaphragm  grows  in  from  the  posterior  margin  of 
the  groove,  and  fi-.ially  the  latter  only  communicates  with 
the  pharvnx  by  a  small  aj)ertiire  in  the  mid-vent-"al  line 
between  the  fourth  pair  of  gill-pouches. 


1 68 


DEVELOPMENT   Of  AMPIIIOXUS. 


Pcripharytis;cal  Ciliated  Bmuis  of  Auimoccctc:. 

Correspondinj^-  with  the  ri[;-ht  and  leit  peripharyngeal 
ciliated  bands  which  we  described  as  proceeding  from  the 
anterior  borders  of  the  endostyle  in  Aniphioxus  there  is 
a  pair  of  ciliated  grooves  in  the  pharyngeal  wall  of  Ammo- 
ccietes  which  proceed  from  the  anterior  lip  of  the  hypo- 
branchial  groove  after  the  latter  has  become  to  a  large 
extent  shut  off  from  the  pharynx  by  the  above-mentioned 
diaphragm.     These  grooves  curve  forwards  and   upwards 


p.b 


Fig.  93.  —  VnimiL;  Aiupliio.xus,  after  tlu'  niftaiiinrpliosis,  IkivIhl;  L'i,i,'lit  i,nll-slils 
on  I'ach  side.     From  the  ri£;ht  side.     (After  Wll.l.l'.V.) 

/./'.  I'eriiiharyngeiil  i)and.  t'.  Velum  ;  shown  sejiarately  below  tiie  main  tisiire, 
with  rudiments  of  four  velar  tentaeles.  ,■.  I-",ndostyle,  extending  backwards  to  the 
level  of  the  fourth  L;iii-siit.     r.m.  I-iigiit  metapleur. 


in  front  of  the  gill-clefts  (after  the  obliteration  of  the  first 
pair  of  gill-pouches),  and  then  proceed  backwards  on  cither 
side  of  the  dorsal  middle  line  of  the  pharynx  as  far  as  the 
commencement  of  the  t.esophagus.  Here  they  appear  to 
cur\e  downwards  again,  and  uniting  together,  extend  for- 
wards as  a  median  ventral  groove  to  the  posterior  lip  of 
the  hypobranchial  aperture. 


AMPI/IOXrS  .LVn   AMMOCaiTES. 


169 


The  last-mentioned  median  ciliated  groove  would  appear 
to  be  unrepresented  in  Amphioxus,  but  the  downward 
curvature  of  the  ciliated  bands  of  the  latter  behind  the  gill- 
slits  can  be  observed  (Fig.  93). 

In  Ammocoetes  the  ciliated  peripharyngeal  grooves, 
where  they  curve  upwards  in  front  along  the  anterior  wall 
of  the  pharynx,  apparently  occupy  the  same  position  which 
was  previously  occupied  by  the  first  pair  of  gill-pouches. 
Since  the  latter  have  already  entirely  disappeared,  there  is 
nothing  in  the  way  of  their  occupying  this  position  (Fig. 
92  C).  In  Amphioxus,  where  the  corresponding  gill-slit 
remains  open  for  a  long  time,  the  peripharyngeal  band  exists 
without  connexion  of  any  sort  with  the  jjortion  of  the  wall 
occupied  by  the  slit,  and  when  the  latter  closes  up,  it  leaves 
no  trace  behind. ^'^ 

Thyroid  Gland. 

When  the  metamorphosis  of  Ammocoetes  into  Petromy- 
zon  takes  place  (which  happens  after  a  larval  existence  of 
some  two  years'  duration),  the  hypobranchial  groove  loses 
all  connexion  with  the  pharynx  and  becomes  broken  up 
by  the  ingrowth  of  connective  tissue  into  a  number  of 
sejiarate  capsules  which  collectively  constitute  the  t/iyroid 
qhiiid  of  Petromyzon, 

The  thyroid  gland  is  one  of  those  enigmatical  ductless 
glands  which  form  such  a  curious  and  constant  feature  of 
t!ie  Vertebrate  organisation. 

There  is  considerable  doubt  as  to  the  specific  physio- 
logical function  which  it  has  to  perform,  but  at  the  same 
time  it  is  a  necessary  factor  in  the  Vertebrate  economy, 
and  is  of  great  importance  from  a  pathological  point  of  view. 

In  the  hitrher  forms  it  is  attached  to  the  lower  side  of 
the  larynx,   and  appears   to    have  received   its    name   on 


imm 


170 


/)£  I  'EL  0PM EN T    OF  AMI'IIIOXI  'S. 


account  of  its  close  proximity  to  tiie  thyroid  cartilage  of 
the  hitter,  the  older  anatomists  assuming  a  functional 
relation  between  the  two  structures. 

We  know  j)erhaps  more  about  the  morphological  than 
about  the  i)hysiological  significance  of  the  thyroid  gland, 
since  it  is  the  vestige  of  the  very  actively  functional  endo- 
style  or  hypobranchial  groove  of  the  Ascidians,  Amphioxus, 
and  Ammocoetes. 


W--^' 


:l.:iili;f 


MorpJiology  of  Club-shaped  Gland  of  Aviphioxiis. 

In  describing  above  the  formation  of  the  second  row  of 
gill-slits  in  Amphioxus,  we  found  that  the  first  secondary 
slit  paired  with  the  second  primary  slit.  It  now  remains 
to  consider  what  has  become  of  the  antimere  of  the  first 
primary  slit. 

The  probability  is  that,  unlike  the  antimeres  of  the  suc- 
ceeding primary  slits,  that  of  the  first  has  not  suffered  a 
retardation  of  development,  but  is  present  from  the  very 
beginning  of  the  larval  development,  although  in  a  some- 
what modified  form.     I  refer  to  the  club-shaped  gland. 

The  club-shaped  gland  fulfils  the  requirements  of  a  gill- 
slit  in  so  far  as  it  opens  at  one  end  into  the  pharynx,  and 
at  the  other  to  the  exterior.  Since,  as  we  have  shown,  the 
morphological  mid-ventral  line  lies  high  up  on  the  right 
side,  immediately  above  the  primary  gill-slits,  it  is  evident 
that  its  anterior  continuatior  would  pass  through  the  en- 
(lostyle  precisely  at  the  point  where  the  latter  is  redoubled 
upon  itself.  But  the  internal  opening  of  the  club-shaped 
gland  lies  above  the  upper  limb  of  the  endostyle,  and 
therefore  it  is  placed  not  only  on  the  actual  right  side  of 
the  larva,  but  in  oi)positit)n  to  the  first  primary  slit,  on  the 
morphological  right  side  as  well. 


AMri/WXiS  AXD  AMMOCiETES. 


I/I 


It  must  be  supposed  that  the  original  gill-slit,  from 
which  the  club-shaped  gland  is  derived,  acquired,  for  some 
reason  or  other,  a  tul)ular  form. 

A  familiar  precedent  for  gill-slits  being  drawn  out  into 
elongated  tubes,  the  effect  of  which  is  to  separate  the  in- 
ternal from  the  external  opening  by  a  long  interval,  is 
presented  by  the  hag-fish,  Mvxinc.  Myxine  also  shows  us 
that,  in  correlation  with  the  canalisation  of  the  gill-slits, 
their  external  apertures  may  enter  into  new  relations  dif- 
fering considerably  from  the  primitive  condition.  As  is 
well  known,  the  elongated  tubular  gill-clefts  of  Myxine  do 
not  open  separately  to  the  exterior,  but  fuse  together  at 
their  distal  extremities,  so  as  to  give  rise  to  a  longitudinal 
duct  on  each  side,  which  opens  to  the  exterior  some  dis- 
tance behind  the  gill-region. 

It  is  only  on  some  such  supposition  as  this  —  namely,  that 
the  external  aperture  of  the  gill-slit  represented  by  the 
club-shaped  gland  of  Amphioxus  has  assumed  new  topo- 
graphical relations  in  correlation  with  the  canalisation  of 
the  original  slit  —  that  its  position  on  the  opposite  (left)  side 
of  the  body  to  the  internal  opening  of  the  gland  is  ren- 
dered intelligible.  The  position  of  the  internal  opening 
furnishes  the  criterion  by  which  to  judge  of  the  primitive 
relations  of  the  original  gill-slit. 

With  the  above  point  of  view,  therefore,  we  may  signal- 
ise the  following  facts  to  prove  that  the  club-shaped  gland 
is  the  antimere  of  the  first  primary  gil;-slit. 

1.  They  arise  simultaneously  in  the   embryo  as  grooves 

in  the  ventral  wall  of  the  pharynx. 

2.  They  come  to  lie  on  opposite  sides  of  the  morphological 

median  line  —  the  first  gill-slit  entirely  so,  and  the 
club-shapeil  gland  in  respect  of  its  internal  opening 
into  the  pharynx. 


1/2 


DEVELOVMEXT   01-   AMrillOXUS. 


I  n'- 


(i 


3.  They  atrophy  and  disappear  simultaneously  during  the 

metamorphosis  of  the  larva. 

4.  No  secondary  gill-slit  ever  arises  to  pair  with  the  first 

primary  slit. 

As  the  stage  represented  in  Fig.  64  marks  such  a  vital 
turning-point  in  the  development  of  the  individual,  being 
the  stage  at  which  the  embryo  becomes  a  larva  and  the 
struggle  for  existence  in  obtaining  independent  nourish- 
ment genuinely  sets  in,  it  is  important  to  be  able  to  define 
it  accurately.  In  view  of  the  above  considerations,  we 
arrive  at  the  conclusion  that  the  larva  is  at  this  sta<re 
possessed  morphologically  of  a  pair  of  gill-slits. 

It  should  be  pointed  out  that  this  opening  stage  of  the 
larval  development  appears  to  be  of  the  nature  of  a  rcst- 
iii!^ phase,  during  which  the  larva  accumulates  energy  for 
future  growth. 

Prceoral  "■  NcpJiridinm"  of  Hat sc lick. 

In  the  larvx  of  Amphioxus  there  is  a  structure  lying  at 
the  base  of  the  notochord  on  the  left  side,  immediately 
above  the  prneoral  pit,  which  we  have  not  yet  consid- 
ered. (Cf.  Figs.  Si  and  82,  x.)  According  to  Hatschek,  who 
first  described  it,  it  arises  in  the  larva  as  a  mesodermal, 
ciliated  funnel  and  canal  in  front  of  the  mouth,  in  the 
region  of  the  first  metamere.  It  lies  in  a  narrow  division  or 
prolongation  of  the  body-cavity,  beneath  the  left  aorta.  (Cf. 
Fig,  76  B.)  At  its  hinder  end  it  opens  into  the  pharynx. 
Hatschek  interprets  this  structure  as  a  nephridium.  Its 
true  physiological,  and  especially  its  morphological,  sig- 
nificance is,  however,  very  perplexing  and  requires  further 
study. 


AMJ'J/IOXLS  A.VD   AMMOCa-:TI:S. 


173 


;s 


Anci'stml  A^itinlnr  of  (lill-siits. 

The  unlimited  number  of  _i;ill-slits  in  the  adult  Amphi- 
oxus  has  led  to  a  ^(u)d  deal  of  controversy  as  to  the  ap- 
proximate number  present  in  the  ancestral  Vertebrate, 
some  authorities    beimr  of   the  opinion    that   Amnhioxus 


presents  the  primitive  condition  in  this  respect,  and 
others  that  the  multiplication  of  gill-slits  in  this  form 
was  a  secondary  phenomenon. 

Sometimes  as  many  as  fourteen  pairs  of  gill-clefts  are 
found  in  a  remarkable  cyclostome  fish  from  the  Pacific, 
allied  to  Myxine,  and  called  /hhl/ostoi/ia*  With  this  ex- 
ception, no  true  fishes,  recent  or  fossil,  are  known  which 
possess  more  gill-slits  than  the  exis  ing  sharks  belonging 
to  the  family  of  the  Xotidixnido.  Of  these  the  genus 
Hcptanchus  possesses  eight  gill-clefts  {i.e.  seven  plus  the 
spiracle)  on  each  side,  and  HcxiDichns  seven.  In  Ammo- 
coetes,  as  wc  have  seen,  there  are  at  one  time  indications 
of  eight  pairs  of  gill-slits.  The  first  pair  of  these,  how- 
ever, never  breaks  through  to  the  exterior,  and  eventually 
disappears,  but  Dohrn  has  shown  that  the  primary  rela- 
tion in  which  the  seventh  pair  of  cranuil  nerves  stands  to 
it,  indicates  that  it  is  the  homologue  of  the  spiracle  of  the 
higher  forms. 

Moreover,  in  the  larval  development  of  Amphioxus 
several  facts  combine  to  produce  the  impression  that  the 
indefinite  number  of  gill-slits  in  the  adult  is  a  secondary 
acquirement.  First  of  all,  there  is  the  series  of  primary 
gill-slits  which,  while  varying  within  narrow  limits,  usually 
numbers  fourteen.  Their  unpaired  unilateral  character  is 
merely  incidental,  as  cxplainetl  above,  and  it  may  be  stated 

*  For  a  recent  account  of  Bdellostoma,  consult  Howard  Avkks,  No.  69, 
bibliography. 


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174 


DEVELOrMEXT   OF  AMPHIOXUS. 


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that  they  are  potentially  paired,  the  first  of  them  in  all 
probability  being  actually  paired  (with  the  club-shaped 
gland). 

In  the  second  place,  after  the  closure  of  a  number  of 
the  primary  slits,  the  so-called  critical  stage  occurs  with 
eight  pairs  of  gill-slits.  This  is  another  resting  phase  in 
the  development,  and  marks  the  turning-point  from  the 
larval  to  the  adolescent  period.  Subsequently  the  addi- 
tion of  new  gill-slits  behind  those  already  present  com- 
mences and  goes  on  indefinitely  throughout  life. 

Counting  in  the  first  pair  of  slits  {i.e.  first  primary  slit 
plus  club-shaped  gland)  which  is  destined  to  atrophy,  we 
must  regard  it  as  probable  that  the  proximate  common 
ancestor  of  Amphioxus  and  the  higher  Vertebrates  was 
characterised  by  the  presence  of  from  nine  to  fourteen 
pairs  of  gill-clefts,  although  it  is  also  probable  that  there 
was  a  variable  tendency  to  add  to  this  number  by  fresh 
perforations. 

NOTES. 

1.  (p.  105.)  It  is  unaccountable  how  there  can  have  been 
conflicting  statements  as  to  the  ejection  of  the  genital  products 
(male  and  female)  through  the  atriopore.  It  was  first  observed  by 
DK  QuAiRr.FACJKs  in  I1S45,  and  his  observations  have  since  been 
fully  confirmeil  by  Pai:i,  Hkkt,  .\.  Wh.lev,  and  E.  H.  Wn^sox.  On 
the  otiier  hand,  both  Kgwai.kvskv  and  Hatschkk  atifirm  that  they 
are  discharged  through  the  mouth.  It  is  to  be  regretted  that  two 
such  eminent  observers  should  have  committed  this  error,  since  it 
is  difficult  to  eradicate  it  from  the  text-books. 

2.  (p.  115.)  The  primitive  endoderm  cells  in  the  neighbour- 
hootl  of  the  neurenteric  canal  ai)parently  retain  an  undifferentiated 
character,  until  the  completion  of  the  myotome-formation.  In  the 
young  embryo  they  are  to  be  observed  in  transverse  section  in  pro- 
cess of  division,  numbers  of  karyokinetic  figures  being  present. 
lUit  the  cells  divide  without  regard  to  the  median  plane  of  sym- 


XOTES. 


'75 


metry,  and  the  recent  researches  of  E.  B.  Wilson  and  Lwokf  lead 
to  the  conclusion  that  the  so-called  mesohlastic  pole-cells,  which 
were  described  by  Hatschek,  have  no  real  independent  existence. 

3.  (p.  123.)  Whether  the  dorsal  and  ventral  fin-spaces  are 
actually  derived  from  the  original  myoccjul,  as  described  by  Hat- 
schek, or  do  not  rather  arise  by  a  splitting  of  an  originally  solid 
thickening  of  the  gelatinous  connective  tissue  which  surrounds 
them,  must  remain  doubtful.  The  cavity  of  the  metapleural  folils 
certainly  arises  as  a  schizoca'l,  i.e.  by  a  hollowing  out  of  a  solid 
thickening.  Even  in  case  the  fin-spaces  also  arise  as  schizoc(cIs, 
Hatschck's  interpretation  of  their  morphological  significance  might 
still  hold  good. 

4.  (p.  123.)  A  transitory  pouch-like  diverticulum  of  the  myo- 
cccl  has  been  observed  in  connexion  with  the  formation  of  the 
sclerotome  in  the  Selachian  embryo  by  Rahl  and  H.  E.  Zikci.kr. 

5.  (p.  129.)  Since  the  work  of  Balfour  on  the  development 
of  Elasmobranch  fishes  (Selachians),  it  has  been  known  that  the 
paired  priemandibular  head-cavities  comrnunicate  with  one  another 
across  the  median  line  in  the  embryo.  The  imi)ortant  results 
obtained  by  the  researches  of  Kupfffr  ( Fetromyzon,  Acii)enser), 
Kasi'schf-NKO  (Selachian),  and  Julia  Pi.vit  (Selachian),  not  only 
established  the  fact  that  the  praimandibular  cavities  arose  essen- 
tially as  anterior  archenteric  pouches  (cf.  Fig.  72),  but  also  that 
the  median  cavity  which  effected  their  communication  across  die 
middle  line,  from  side  to  side,  arose  by  constriction  from  the  front 
end  of  the  archenteron  (using  the  latter  term  with  some  latitude), 
and  that,  therefore,  the  laiiofi  of  the  right  and  left  pranuDuiibular 
cavities  in  the  embryo  of  the  craniate  Vertebrates  is  primary,  and 
not  secondary,  as  was  previously  supposed. 

I'or  an  excellent  historical  and  critical  summary  of  our  knowl- 
edge of  the  origin  of  the  head-cavities  in  the  craniate  Vertebrates, 
the  reader  may  consult  Frorikp.     (See  bibliography.) 

6.  (p.  130.)  The  ciliation  of  the  ectoderm  in  the  larva  of 
Amphioxus  continuing,  as  it  does,  long  after  the  muscles  have  been 
fully  differentiated,  and  when  the  cilia  are  therefore  no  longer 
required  for  purposes  of  locomotion,  should  be  especially  noted  as 
evidence  of  a  very  archaic  organisation. 

We  shall  find  in  the  last  chapter  that  the  possession  of  a  ciliated 


7/4^ 


176 


DEVELOPMEmXT   OE  AMPI'IOXUS. 


'■  t 
( 


!■  1  -'. 


it. 


ectoderm  is  a  prime  characteristic  of  Balanoglossus  and  many  of 
the  lower  worms  (e.g.  Nemcrtines).  In  none  of  the  craniate 
Vertebrates  is  the  ectoderm  at  any  time  ciliated. 

7.  (p.  134.)  The  exact  st'fre  at  which  the  club-shaped  gland 
reopens  into  the  pharynx  must  remain  an  open  question.  It  is, 
very  probably,  subject  to  a  good  deal  of  variation  in  this  respect, 
occurring  now  earlier,  now  later.  Experiments  to  determine  the 
physiological  role  of  this  gland  are  much  needed. 

<S.  (p.  143.)  In  accordance  with  Dohrn's  conception  of  the 
lirincii)le  of  the  change  of  function  {^Das  Princip  des  Functions- 
wcchsels),  the  number  and  nature  of  the  o''gans  of  the  Vertebrate 
body,  which  have  been  interpreted  as  modified  gill-clefts,  are  truly 
astonishing.  First  and  foremost,  Dohrn  supposed  that  the  Verte- 
brate mouth  arose  by  the  fusion  of  two  gill-slits  across  the  middle 
line,  the  old  Annelid-mouth,  which  perforated  the  central  nervous 
system,  having  been  lost.  A  great  many  forcible  arguments  have 
been  brought  forward  in  support  of  this  hypothesis.  Dohrn  him- 
self would  probably  admit  that  it  is  only  tenable  on  his  further 
hypothesis  that  Amphioxus  is  a  form  which  has  undergone  a  retro- 
gressive evolution  from  the  craniate  Vertebrates.  This  was  a 
better  hypothesis  than  that  of  Semper,  who,  perceiving  that 
Amphioxus  would  not  fall  in  with  the  Annelid-theory,  declared, 
"  er  sei  kein  Wirbelthier  ;  also,  auch  kein  Fisch." 

Besides  the  mouth,  many  other  structures  have  similarly  been 
referred  back  to  modified  gill-slits,  among  which  may  be  mentioned 
the  nose,  hypophysis,  thyroid  gland,  lens  of  the  eye,  and  the  anus. 
^Cone  of  these  comparisons  is  supported  by  the  facts  of  develop- 
ment and  anatomy  of  either  Amphioxus  or  the  Tunicates,  while 
most  of  them  would  appear  to  be  definitely  disproved  by  these 
facts. 

9.  (p.  147.)  Since  the  right  metapleural  fold  bends  round  to 
the  median  ventral  line  ot  the  snout,  as  shown  in  Fig.  38,  and 
since,  further,  at  a  later  period,  the  right  half  of  the  oral  hood  is 
similarly  continued  round  the  front  end  of  the  body  into  the 
dorsal  fin,  it  is  clear  that  the  right  half  of  the  oral  hood  must 
arise  essentially  in  continuity  with  the  right  metapleur.  On  the 
contrary,  the  left  half  of  the  oral  hood  arises  entirely  independently 
of  the  left  metapleur.     It  is  possible  that  this  discontinuity  of 


¥  \. 


NOTES. 


177 


development  of  the  left  half  of  the  oral  hood  and  the  left  meta- 
pleur  has  been  secondarily  brought  about. 

10.  (p.  150.)  The  study  of  transverse  sections  has  led  me  to 
the  conclusion  that  the  backward  extension  of  the  endostyle  is 
effected  by  interstitial  growth,  and  not  by  the  conversion  of  the 
cells  which  form  the  primary  floor  of  the  pharynx  into  endostylar 
elements.  These  cells  are  probably  disintegrated  and  absorbed 
by  the  endostyle  as  it  grows  backward. 

11.  (p.  153.)  For  a  comparison  between  the  perigonadial 
cavities  of  Amphioxus  and  the  mesonephric  tubules  of  the 
craniates  the  reader  should  consult  Boveri's  original  memoirs. 
(See  bibliography.) 

12.  (p.  159.)  The  following  definition  of  the  so-called  bio- 
genetic law  of  recapitulation  (Haeckel's  biogentisches  Grund- 
gesetz)  will  explain  the  meaning  of  Haeckel's  terms  "  cenogenesis  " 
and  "  palingenesis."  According  to  this  law  :  The  development  of 
the  individual  {ontoi^eny^  is  a  compressed  summary  of  the  gradual 
modifications  which  have  resulted  in  the  evolution  of  the  species, 
or  type  (///j^^'-<?«v=  Stammesgeschichte)  ;  this  recapitulation 
(summary,  or  Auszug)  of  the  phylogenetic  stages  in  the  ontogeny 
is  the  more  perfect  according  as  the  ancestral  development 
(Palingenesis,  Auszugsentwicklung)  has  been  the  less  disturbed 
or  fiilsified  through  secondary  or  "  recent "  adaptation  (ceno- 
genesis, Storungsentwickelung)  of  the  embryo  or  larva  to  a  new 
environment. 

13.  (p.  162.)  The  explanation  of  the  asymmetry  of  the  larva 
of  Amphioxus  given  in  the  text  was  first  suggested  by  me  in  1891. 
It  may  be  well  to  state  that  it  has  not  as  yet  received  very  general 
recognition  in  the  more  recent  literature  on  the  subject.  It  was, 
however,  fortunate  enough  to  receive  the  endorsement  of  the  late 
Professor  Miiaks  Marshall  in  his  text-book  of  Vertebrate  Em- 
bryology. When  the  pelagic  larvae  of  Amphioxus  are  confined  in 
glass  jars,  after  a  certain  lapse  of  time  they  sink  to  the  bottom, 
like  all  other  pelagic  organisms.  When  they  arrive  at  the  bottom, 
they  fall  over  on  to  one  side,  owing  to  a  physical  impossibility  to 
rest  in  any  other  position,  just  as  was  described  above  for  the 
adult.  It  ought  not  to  require  to  be  emphasised  that  their  inci- 
dentally lying   on  one  side   is   not  due  vo  a  pressing  desire  or 


m 


1 


hi! 


ni  !iS^!- 


M 


178 


DEVELOr.MENT   OF  AMPIllOXUS. 


instinct  to  assume  that  position,  but  rather  because  they  cannot 
help  it.  It  is  apparently  in  consequence  of  a  misunderstanding 
of  ihis  observation  that  Korschelt  and  Hkider  ascribe  the  larval 
asymmetry  of  Amphioxus  to  the  same  causes  which  brought  about 
the  asymmetry  of  the  Pleuronectidae.  Another,  and,  as  it  appears, 
a  still  more  impossible  view,  has  recently  been  expressed  by  van 
WijHE.  According  to  van  Wijhe,  the  left-sided  mouth  occupies  its 
normal  and  primitive  position  in  the  larva  of  Amphioxus,  and  in 
that  position  it  represents  a  gill-slit,  whose  antimore  is  the  club- 
shaped  gland.  Van  Wijhe  arrived  at  this  view  as  a  result  of  his 
very  important  discoveries  as  to  the  musculature  and  innervation 
of  the  adult  mouth.  These  discoveries  may  be  summarised  as 
follows :  — 

1.  The  outer  muscle  of  the  oral  hood  represents  the  anterior 
continuation  of  the  left  half  only  of  the  transverse  and  subatrial 
muscles. 

2.  The  inner  nerve-plexus  of  the  oral  hood  is  for. tied  on  both 
sides,  exclusively  from  nerves  which  arise  from  the  leit  side  of  the 
central  nervous  system. 

3.  The  velum  is  innervated  entirely  from  nerves  of  the  left  side. 

From  these  observations  van  Wijhe  concludes  that  the  mouth  of 
Amphioxus,  even  in  the  adult,  is  essentially  an  organ  of  the  left 
side,  and  is  neither  homologous  with  the  Ascidian  nor  with  the 
craniate  mouth. 

It  would  seem,  however,  that  the  more  obvious  and  justifiable 
interpretation  of  these  facts  is  that  the  asymmetrical  musculature 
and  innervation  described  by  van  Wijhe  are  merely  the  partial 
persistence  in  the  adult  of  the  more  complete  asymmetry  of  the 
larva. 

Van  Wijhe's  observations,  therefore,  do  not  affect  the  que.stion 
of  the  cause  of  the  asymmetry  in  any  degree. 

14.  (p.  165.)  As  first  shown  by  Dohrn,  the  hypophysis  of 
Ammocoetes  first  arises  from  the  roof  of  the  stomodoeum,  from 
which  it  is  subsequently  removed  to  the  dorsal  surface  of  the  head 
by  the  enormous  development  of  the  upper  lip. 

15.  (p.  169.)  The  ciliated  tracts  in  the  pharynx  of  Ammo- 
coetes were  first  dci.cribed  and  figured  by  Anton  Schneider  in 


XOTES. 


179 


1879.  In  1886  DoHRN  thought  he  had  proved  that  the  anterior 
portion  of  them,  which  bends  upwards  on  either  side  of  the 
pharynx  and  forms  the  peripharyngeal  grooves,  represented  the 
last  traces  of  the  aborted  first  pair  of  gill-clefts.  Although  they 
appear  at  the  place  which  was  formerly  occupied  by  these  rudi- 
mentary gill-pouches,  yet,  according  to  Dohrn's  own  account,  they 
do  not  appear  until  after  the  gill-pouches  have  completely  flatteneti 
out.  Under  these  circumstances,  but  above  all,  in  view  of  the 
relations  of  the  homologous  peripharyngeal  bands  in  Amphioxus 
which  exist  both  before  and  after  the  disappearance  of  the  first 
pair  of  gill-clefts  {i.e.  first  primary  gill-cleft  and  club-shaped 
gland),  it  must  be  assumed  that  Dohrn's  interpretation,  though 
most  natural,  was  nevertheless  somewhat  at  fault. 


m 


W'i 


IV. 


THE   ASCIDIANS. 

The  Ascidians,  Tunicates,  or  sea-squirts,  as  they  arc 
indifferently  called,  constitute  one  of  the  most  clearly 
defined  and  yet  most  heterogeneous  groups  of  animals 
which  it  is  possible  to  imagine.  There  is  a  great  variety 
of  families,  genera,  and  species  occurring  all  the  world 
over,  and  in  all  depths  of  the  ocean  from  the  tide-marks 
to  the  profoundest  depths. 

Most  of  them  are  sedentary  animals,  remaining  fixed 
all  their  lifetime  on  one  spot,  whether  attached  to  rocks, 
stones,  shells,  or  sea- weeds,  from  which  they  are  incapable 
of  moving.  There  are,  however,  several  very  extraordi- 
nary genera  of  Ascidians  which  swim  or  float  about  per- 
petually in  the  open  ocean,  and  have  become  adapted  in 
the  extremest  manner  to  a  purely  pelagic  environment. 
These  pelagic  Ascidians  have  become  so  modified  in  adap- 
tation to  their  oceanic  existence,  and  their  development 
diverges,  as  a  rule,  so  much  from  the  normal,  that  they 
will  hardly  enter  at  all  into  the  present  discussion,  with 
the  exception  of  one  family,  the  AppcndicularicB. 

Just  as  there  are  two  kinds  of  sessile  Ascidians,  simple 
and  compound  or  colonial,  so  there  are  two  analogous  kinds 
of  pelagic  Ascidians.  In  some  of  the  latter,  however, 
where  there  is  an  alternation  of  generations,  one  genera- 
tion, namely,  the  asexual  generation,  is  a  solitary  form, 
while  the  sexual  generation  is  a  colonial  form,  as,  for 
example,  the  solitary  Salpa  and  the  chain-Salpa. 

i8o 


ANATOMY  AXD   DEVELOPMENT. 


I8l 


For  convenience,  the   Ascidians,  as  a  whole,  may  be 
arranged  as  follows:  — 

SLSSILL   ASCIDIANS. 
SiMi'Lii.  Colonial. 


eg.  .Iscidia. 
rhallusia. 
Ciona. 
Molk^nla. 
Cynthia. 


SiMi'i.i;. 
e.g.  .IppCHiiicularia. 


e.g.  Clave  I  til  a. 
liotryllits. 
Aiiiaroitcium. 
Di si  a  pita. 
Didemniim. 


PELAGIC   ASCIDIANS. 

Colonial 

(or  capable  of  producing  a  colony 

by  budding), 
e.g.  Pyrosoma. 

Sal  pa. 

Doliolnin. 


The  compound  sessile  Ascidians  consist  of  colonies  of 
individuals  or  ascidiozooids  produced  by  budding  from  a 
paient  individual.  Such  colonies  are  often  brilliantly 
coloured  and  of  massive  proportions,  as  Amarouciiim  and 
Fragarinm.  Others  form  thin  encrusting  expansions  on  the 
surfaces  of  marine  plants  and  shells,  as  Botrylliis  and  Lcpto- 
cliuHid.  In  others,  again,  the  individuals  are  entirely 
separate,  except  at  the  base,  where  they  are  connected 
together  by  a  common  creeping  stolon  from  which  new 
buds  are  periodically  produced,  as  Clavclina  and  Perophora. 

STRUCTURE  OF   A   SIMPLE   ASCIDIAN. 

Test,  Mantle,  Atriuvi,  Branchial  Sac. 

The  simple  or  solitary  Ascidians  which  do  not  produce 
buds,  present  hardly  less  striking  differences  among  the 
different  families  than  do  the  compound,  but  their  general 
shape  is  much  more  uniform. 


^m 


182 


77//r  ASCIDIANS. 


*li   ^ 


Li^^ 


iU  ' 


i.# 


M 


41 


An  average  simple  Ascidian,  as  Phallusia  or  Cynthia, 
has  been  aptly  compared  to  a  leather  bottle  provided  with 
two  spouts.  The  spouts  occur  in  the  form  of  two  funnel- 
like prominences  projecting  from  the  surface  of  the  body 
and  beariuj^^  at  their  free  extremities  .he  inciirrent  or  buc- 
cal and  cxcnvrcut  or  cloacal  apertures  respectively,  the 
latter  usually  occurring  at  a  lower  level  than  the  former. 

The  most  prominent  and,  apart  from  the  two  apertures, 
the  only  external  feature  of  a  simple  Ascidian,  is  the  char- 
acteristic tunic  or  test  which  surrounds  the  whole  body.  As 
a  rule,  all  Ascidians  of  whatever  kind  possess  this  external 
tunic,  and  it  is  one  of  their  chief  diagnostic  characters. 

According  to  '.he  species  this  test  may  be  of  a  cartilagi- 
nous, coriaceous,  fibrous,  or  membranous  consistency, 
usually  opaque,  but  sometimes  hyaline  and  transparent,  as 
in  Corclla,  Salpa,  etc.  Its  outer  surface  may  be  smooth, 
wrinkled,  or  rough,  capillatcd,  papillated,  or  mammillated. 
In  1845  Kakl  Schmidt  made  the  discovery  that  the  test 
of  the  Ascidians  was  largely  composed  of  the  substance 
which  forms  the  cell-walls  in  plant  tissues  ;  namely,  cellu- 
lose. When  treated  with  the  proper  chemical  reagents,  it 
gives  the  cellulose-reaction.  This  is  interesting  as  show- 
ing the  fundamental  identity  of  protoplasm  whether  it 
occurs  in  animal-  or  in  plant-cells,  since  in  both  cases  it 
is  capable  of  depositing  cellulose. 

Judging  by  external  appearances  an  ordinary  Ascidian 
resembles  notl.ing  so  little  as  Amphioxus,  and  yet  it  is 
probably  more  closely  related  to  the  latter  than  is  the 
lamprey  larva,  Ammocoetes,  whose  external  resemblance 
to  Amphioxus  is  incomparably  greater. 

It  is  only  in  its  internal  organisation  that  we  meet  with 
structures  which  remind  us  strongly  of  corresponding 
parts  in  Amphioxus. 


ANA  J  CM y  AND  DEVE/.O/WfEXT. 


183 


11 


A  schematic  representation  of  a  dissection  of  a  typical 
Ascidian  after  I'rofessor  W.  A.  Hekdman,  whose  reports  on 
the  Ascidians  collected  .luring  the  voyage  of  H.  M.  S. 
Challenger  have  done  so  much  to  advance  our  knowledge 
of  the  group,  is  given  in  Fig.  94.  The  greater  part  of  the 
thick  cartilaginoid  test  (also  called  tunic,  outer  mantle,  or 
cellulose  mantle),  /,  is  supposed  to  be  removed  from  the 
right  side,  and  its  cut  edge  can  be  traced  all  the  way  round. 
Below  the  test  comes  the  inner  or  muscular  mantle,  ;;/, 
which  is  the  true  body-wall,  to  which  the  external  tunic  is 
secondarily  superadded.'  The  muscular  mantle  is  limited 
externally  (below  the  test)  by  the  epidermis,  and  beneath 
the  latter  are  the  interlacing  muscle-fibres  which  compose 
the  bulk  of  the  inantle. 

Beneath  the  mantle  is  an  extensive  cavity  surrounding  to 
a  large  extent  the  viscera.  This  is  the  pi-rihrntichial  or 
atriixl  cavity  which  communicates  with  the  exterior  by  the 
atrial  or  cloacal  aperture,  at.s. 

The  mouth,  or.s,  leads  into  the  />/itirym-  or  hmncJdal  sac, 
ph,  which  is  of  surprising  dimensions,  and  stretches  nearly 
to  the  posterior  end  of  the  body.  The  walls  of  the  bran- 
chial sac  are  perforated  by  innumerable  gill-openings,  the 
so-called  stij^mata,  arranged  in  successive  transverse  rows, 
through  which  the  water  which  enters  at  the  mouth  passes 
out  of  the  sac  into  the  atrial  cavity. 


I 


Dorsal  Lamina,  Endostylc,  and  PcripJuDyngcal  Ba)id. 

On  cutting  through  its  right  wall  we  open  into  the 
cavity  of  the  branchial  sac  along  the  dorsal  side  of  which 
a  fold  is  seen  projecting  freely  into  the  cavity,  the  so-called 
dorsal  lamina  corresponding  to  the  dorsal  groove  in  the 
pharynx  of  Amphioxus,  while  along  its  ventral  side  is  a 


J/^- 


w 


184 


7'//A   ASC/DIAAS. 


"-!  • 


Vu 

'■'i 


m 


w 


.1, 


W. 


\     r:y 


at.s.... 


an 


Fig.  94.  —  Diagram  of  a  dissection  of  Ascidia,  from  the  right  side.  (After 
Hkriiman.) 

The  peribranchial  cavity  is  indicated  by  the  black  shading. 

an.  Anus.  at..<!.  Atrial  siphon,  e.g.  Cerebral  ganglion,  beneath  which  is  the 
subncural  gland  and  its  duct.  ci.l.  Dorsal  lamina,  end.  Endostyle.  g.  Gonad. 
g.d.  Genital  duct.  ////.  Intestine.  111.  Muscular  mantle,  oes.  Aperture,  leading 
from  branchial  sac  into  tL'Sophagus.  ot  .s.  Buccal  siphon,  ph.  Branchial  sac. 
.?/.  Stomach.  /.  Test  or  cellulose  mantle,  fn.  Buccal  or  coronary  tentacles. 
ty.  Typhlosole ;  internal  fold  of  intestinal  wall,  to  increase  the  digestive  surface. 


'Mj 


.■/.\'./7Ul/r  AND   DEl'ELOl'MENT. 


1S5 


hW 


wcU-ilcfinccl  groove  with  white  ^li.stenin«;  walls,  which  is 
the  CHiiostylc.  The  groove  of  the  endostyle  is  deeper  here 
than  in  Amphioxus,  but  its  epithelial  walls  have  the  same 
histological  differentiation,  with  the  two  rows  of  glan>' 
cells  on  each  side  of  the  middle  line,  the  latter  being 
occupieil  by  a  median  group  of  cells  carrying  very  long 
cilia.  The  food  which  enters  the  mouth  together  with  the 
water  does  not  pass  out  of  the  pharynx  into  the  atrial 
chamber,  but  is  caught  up  by  the  slime  secreted  by  the 
endostyle  and  is  then  z-AXx'\^f\  fori^Hxrds  along  the  endostyle, 
and,  having  arrived  at  the  anterior  extremity  of  the  latter 
at  the  base  of  the  buccal  tube,  is  carried  round  along  a 
circular  ciliated  groove  which  surrounds  the  base  of  the 
mouth  at  the  entrance  to  the  branchial  sac,  until  it  reaches 
the  dorsal  side  of  the  animal,  when  it  is  led  backwards  by 
the  ciliary  action  of  the  cells  of  the  dorsal  lamina  in  the 
form  of  a  cord  of  slime  in  which  the  food-particles  (micro- 
scopic organisms,  vegetable  debris)  are  imbedded. 

The  ciliated  groove  round  the  base  of  the  buccal  tube 
connecting  the  anterior  extremity  of  the  endostyle  with  the 
dorsal  lamina  is  known  as  the  pcnpharyn<^cal  band  or 
pcricoronal  groove.  We  have  already  made  the  acquaint- 
ance of  the  homologue  of  this  structure  both  in  A.mphi- 
oxus  and  in  Ammocoetes.  It  forms  a  complete  circle 
round  the  base  of  the  buccal  tube  and  is  indicated  in 
Fig.  94  by  the  black  line  which  limits  the  pharyngeal 
wall  anteriorly.  It  is  still  better  shown  in  Fig.  96,  which 
represents  a  young  individual  of  Clavelina. 

The  cord  of  slime  containing  the  food  passes  backwards 
along  the  dorsal  lamina  to  the  opening  of  the  cesophagus, 
which  lies  near  the  posterior  end  of  the  branchial  sac,  in 
the  dorsal  middle  line,  through  which  it  passes  into  the 
stomach.     The  dorsal  lamina  is  continued  to  one  side  of 


i.i 


//. 


>^ 


-> 


' ' ' )'    i 


Si!    .' 


1 86 


77//r  ASC/D/AA'S. 


the  oesophageal  aperture,  as  a  low  ridge,  which  joins  the 
posterior  extremity  of  the  endostyle.* 

Viscera/  Anatomy. 

Except  in  its  most  anterior  region,  the  dorsal  border  of 
the  pharynx  lies  freely  in  the  atrial  chamber.  On  the 
contrary,  along  its  ventral  border,  throughout  the  whole 

length  of  the  endo- 
style, it  is  attached  to 
the  muscular  mantle. 
In  other  words,  the 
right  and  left  halves 
of  the  atrial  cavity  are 
continuous  round  the 
dorsal  side  of  the 
pharynx,  but  are 
separated  from  one 
another  ventrally  by 
the  concrescence  of 
the  endostyle  with 
the  mantle.  (Cf.  Fig. 
95.)  In  Amphioxus, 
as  we  have  seen,  the 
opposite  condition  ob- 
tains. There,  the  dor- 
sal wall  of  the  pharynx 


•r.o 


F'g-    95-  —  Diagrammatic  transverse    section    is    closely   applicil    to 
throutjli  the  middle  of  tlie  body  of  ,-/jc/'<j'/d:.     (After      ■  f       I         1  1  '1 

Hk.KKMAN.)       The  muscular   mantle  is  indicated    '^'"'^    notOChOrcl,    WnilC 
by  the  liLuk  shading;. 

(7.  rfiibr.mehial  cavity  traversed  by  numerous 
vascular  trabecuhv,  through  which  the  blooti  flows 
into  the  bra.ichial  bar*  hr.s.  Brancliial  sac. 
b.r>.  "  Blood-vessels."  ii.t.  Dorsal  lamina,  e.  Endo- 
style. ec.  Keto<lerm.  ,.i'.  Cionad.  ^"d.  Double 
genital  iluct.  /.  Intestine,  with  'viihlosole.  /■.  Rec- 
tum.    '■.('.  Renal  vesicles.     A  'lest. 


the    endostylar   tract 

*  Compare  the  above  witli 
the  description  of  the  course 
of  the  ciliated  tracts  in  the 
pharynx  of  Aniinoc(etes, 
given  on  p.  1C8. 


ANATOMY  AND  DEVELOPMENT. 


187 


is  free,  so  that  the  right  and  left  halves  of  the  atrial 
cavity  are  continuous  ventrally,  instead  of  dorsally. 

In  order  to  see  the  stomach  and  intestine,  it  is  necessary 
to  cut  through  the  left  wall  of  the  pharynx,  since  the  vis- 
cera lie,  at  least  in  the  genus  Ascidia  (or  Phallusia),  on 
the  left  side  of  the  pharynx.  It  should  be  pointed  out 
that  the  topographical  arrangements  vary  considerably 
among  the  different  genera  of  Tunicates.  In  Clavelina, 
for  example,  the  viscera  lie  behind  the  pharynx,  as  shown 
in  Fig.  96. 

On  the  left  side  of  the  pharynx  (Fig.  94)  the  short 
oesophagus  leads  into  the  dilated  stomach,  which  again 
narrows  down  to  the  looped  intestine,  and  finally  the  lat- 
ter bends  sharply  forwards  into  the  rectum,  which  opens 
by  the  anus  into  the  atrial  cavity,  the  excrement  being 
carried  to  the  exterior  by  the  constant  stream  of  water 
which  flows  out  through  the  atrial  or  cloacal  aperture. 

Instead  of  being  straight,  as  in  Amphioxus,  the  aliment- 
ary canal  is  here  tioubled  round  upon  itself.  This  U-shaped 
character  of  the  alimentary  canal  of  Ascidians  is  shown 
with  great  clearness  in  the  case  of  Clavelina  (Fig.  96), 
where  there  are  no  secondary  convolutions  in  the  course 
of  the  intestine. 

The  Ascidians  are  one  and  all  JicnnapJiroditc,  and  the 
reproductive  glands  frequently  lie  between  the  loops  of 
the  intestine,  while  two  ducts,  oviduct  and  vas  deferens^ 
which  often  present  the  appearance  of  a  single  duct  with 
a  double  lumen,  proceed  forwards  by  the  side  of  the  rec- 
tum, to  open  into  the  cloacal  region  of  the  atrial  cavity 

near  the  anus  (Fig.  94, a*"  ^^^^  A'''0- 

The  ovary  and  testis,  though  quite  separate  in  the  adult, 
originate,  according  to  ihe  account  given  by  the  Belgian 
zoologists,  Edouaku  v.\n  Bknehen  and  Cu.\rles  Julin, 


'•>. 


188 


77//i   ASCIEIANS. 


•A 


% 


\m 


from  a  common  centre  of  formation,  which  subsequently 
undergoes  a  division  into  two  portions,  one  of  which  be- 
comes the  ovary,  and  the  other  the  testis.  Similarly  the 
oviduct  and  vas  deferens  are  derived  by  division  of  a 
primarily  single  structure,  which  arises  in  continuity 
with,  and  in  fact  as  an  outgrowth  from,  the  primitive 
sexual  gland. 

In  spite  of  their  hermaphroditism,  it  would  appear  that 
not  all  the  Ascidians  are  self-fertilising,  although  many,  if 
not  most  of  them,  are.  In  some  cases  it  is  supposed  that 
in  different  individuals  the  male  and  female  organs  attain 
maturity  at  different  times,  so  that  in  a  given  individual, 
when  the  ovary  is  ripe  the  testis  is  unripe,  so  that  it  must 
be  fertilised  from  another  individual,  in  which  the  testis  is 
ripe,  but  the  ovary  unripe,  and  so  on. 

Nervous  System  and  Hypophysis. 

(^Neurohypophysial  System.) 

The  central  nervous  system  of  an  Ascidian  usually  bears 
a  ridiculously  small  proportion  to  the  bulk  of  the  organ- 
ism. Its  main  constituent  is  a  ganglion  which  lies  im- 
bedded in  the  thickness  of  the  mantle,  between  the  oral 
at  d  the  atrial  siphons,  the  two  latter  structures  being 
innervated  by  nerves  proceeding  from  the  ganglion.  As 
belonging  to  the  central  nervous  system  must  also  be 
mentioned  a  solid  nerve-cord  which  runs  along  the  dorsal 
border  of  the  branchial  sac  from  the  cerebral  ganglion 
to  the  visceral  region  (Fig.  96).  This  was  discovered  by 
van  Beneden  and  Julin,  and  is  derived  from  a  persistent 
portion  of  the  central  nervous  system  of  the  larva. 

Beneath  the  cerebral  ganglion  is  a  lobulated  glandular 
organ  known  as  the  siibnciiral  gland.     It  is  provided  with 


lit     E... 


AAATOiMY  AND  DEVELOPMENT. 


189 


a  duct  which   runs  forward  and  opens  at  the  end  of  a 

ciliated  funnel-stiaped  dilatation  into  the  branchial  sac  at 

the  base  of  the  buccal  tube 

(Figs.    94,   96,    and   97)    in 

front  of  the  peripharyngeal 

band. 

The  branchial  opening  of 
the  duct  of  the  subneural 
gland  appears  primarily  as 
a  simple  circular  orifice,  but 
it  does  not  usually  retain 
this  character  in  the  adult. 

Generally  it  assumes  a 
crescentic  form  by  the  in- 
curving of  its  anterior  or 
posterior  lip,  and  then  in 
many  cases  the  horns  of  the 
crescent  so  formed  become 
coiled  over  and  over  con- 
centrically,  and   usually   in 

,  .,  Fig.  96. —Young  Clavelina,  shortly 

approximately        tne        same    3^^^  the  metamorphosis,  from  the  right 

side.  (After  VAN  Bknkuen  and  Ji  lin.) 
at.  Atrial  opening,  at.c.  Atriai  cav- 
ity, b.s.  Blood-sitius.  enJ.  Endostyle. 
ep.  Epicardium ;  outgrowth  from  bran- 
chial sac  behind  endostyle,  which  grows 
down  into  the  creeping  stolon,  forming 
a  septum  in  the  latter,  and  being  the 
chief  element  in  the  production  of  buds. 
f.  Lobes  of  the  fixing  o^-gan,  which  give 
rise  to  the  creeping  stolon,  j^.  Ganglion. 
^.s.  Stigmata,   h.  jleart.    /ly.  Hypophysis 


plane,  so  that  the  lips  of 
the  aperture  assume  a  very 
complicated  appearance  and 
constitute  the  so-callcrl  dor 
sal  tubercle  (Fig.  97). 

It  has  taken  a  long  time 
and    the   work    of    a   great 


(dorsal  ti.berclc).  /;;/.  Intestine.  w. 
many  zoologists  to  achieve  Mouth,  .w.  CEsophagus.  />.i«.  Periphar- 
Olir         r>rpspnt         knowlerlo-o    >'"'-"'•''  '*'''''''•    ^"-    I'er'cardium.    /.    Re- 

our       present       Knowledge   ^^j^^  ^^  ,^5,  withdrawn  into  the  body. 

(which       is       by      no       means    ^•"-  visceral  nerve. 

complete)    of   the  subneural  gland  of  Ascidians  and    its 
duct. 


■^>. 


0^ 


Im 


fc; 


i 

it 

|^h|J^2J/  i 

190 


THE  ASCIDIAA6. 


Fig.  97-  —  Hypophysis  of  Phallusia 
mentiila,  prepared  out  and  seen  from  the 
inside.     (After  JULIN.) 

g.  Snbneural  gland,  above  which  may 
be  seen  the  outline  of  the  ganglion  and  its 
nerves,  d.  Duct  of  the  subneural  gland. 
/.  Dorsal  tubercle,  the  opening  of  the 
hypophysis  into  the  branchial  sac.  The 
actual  opening  is  indicated  in  blaek. 
pc.  Peripharyngeal  groove.  ep.  Epi- 
branchial  groove,  d.l.  Dorsal  lamina, 
slightly  displaced,  to  show  the  duct  of  the 
subneural  gland  above  it. 

N.B.  —  In  this  species,  the  atrial  and 
buccal  siphons  are  widely  separated,  and 
the  duct  of  the  subneural  gland  is  very  long. 


The  dorsal  tubercle  was 
discovered  by  the  celebrated 
Savigny  in  '1816,  and  was 
for  a  long  time  supposed  to 
be  an  independent  sense- 
organ  of  an  olfactory  nature. 
The  subneural  gland  was 
detected  not  as  a  gland,  but 
as  an  enigmatical  structure 
lying  below  the  brain  by 
the  English  naturalist  Han- 
cock in  1868.  Its  glandular 
character  was  demonstrated 
by  Nassonoff  and  (Jssow 
in  1874-75,  the  last-named 
author  showing  its  connex- 
ion by  means  of  the  duct 
with  the  dorsal  tubercle.  In 
1 88 1  JuLiN  produced  an 
admirable  memoir  on  the 
subneural  gland  and  its  duct, 
and  strongly  urged  its  ho- 
mology with  the  pituitary 
body  or  hypophysis  cerebri 
of  the  higher  Vertebrates. 
The  same  suggestion  was 
made  in  a  more  tentative 
form  in  the  same  year  by 
Balfour.  We  shall  have 
to  consider  this  question 
later.  Suffice  it  to  say  at 
present  that  Julin's  sugges- 
tion   has   been  accepter'  to 


AX.lTOJ/y  AND  DEVELOIWIENr. 


191 


if 


the  extent  that  the  subneural  organ  of  the  Ascidians  is 
frequently  spoken  of  as  the  hypophysis. 

Circulatory  System. 

With  regard  to  the  circulatory  system  the  Ascidians 
differ  markedly  from  Amphioxus  in  the  possession  of  a 
well-defined  heart  which  lies  in  a  distinct  pericardium. 
The  heart  lies  ventrally  and  usually  in  the  neighbourhood 
of  the  stomach.  (Cf.  Fig.  96.)  Its  wall  is  muscular,  but 
consists  only  of  a  single  layer  of  cells  whose  deeper  portions 
{i.e.  towards  the  cavity  of  the  heart)  are  drawn  out  into 
striated  muscular  fibres,  while  the  outer  portions  of  the 
cells  containing  the  nuclei  project  into  the  cavity  of  the 
pericardium. 

There  is  therefore  no  true  endothelial  lining  to  the  heart, 
and  the  cells  which  build  up  its  wall  offer  a  most  interest- 
ing example  of  epithelio-muscular  tissue,  as  was  first  pointed 
out  by  Edouard  v^^n  Beneden.  This  type  of  muscular  tis- 
sue, in  which  the  muscle-fibres  occur  as  basal  prolonga- 
tions of  cells  which  still  retain  their  epithelial  character,  is 
found,  as  is  well  known,  in  the  case  of  the  body-muscles  of 
the  Nematode  or  thread-worms,  and  is  above  all  character- 
istic of  the  Coelenterata  (Hydroids  and  Medusas). 

There  are  no  true  blood  vessels  in  Ascidians,  but  the 
passages  along  which  the  blood  percolates  are  merely 
lacunae  in  the  connective  tissue  and  musculature  of  the 
body  and  between  the  viscera.  They  are  not  lined  by  an 
endothelium,  and  are  more  correctly  described  as  blood- 
siuus^s.  They  are  often  irregular  in  their  outline,  as  shown 
in  the  transverse  section  represented  in  Fig.  95,  but  often 
again  they  simulate  the  appearance  of  true  blood-vessels, 
as  in  the  case  of  those  branches  which  pass  from  the 
man'.le  into  the  substance  of  the  test,  as  well  as  the  tubes 


m 


!|' 


;^/i 


/'i^ 


w 


■5  I 
I 


1  ,'l 


m 


:j 


192 


7//£  ASCIDI.tXS. 


which  traverse  the  wall  of  the  branchial  sac  in  every 
direction. 

In  the  second  chapter  it  was  pointed  out  that  the 
Vertebrate  heart  arose  as  a  specialisation  of  a  portion  of 
the  primitive  sub-intestinal  blood-vessel  whose  calibre  was 
originally  uniform  throughout,  and  that  in  Amphioxus  the 
cardiac  region  of  the  vascular  system  retains  its  primitive 
tubular  character. 

Very  different  is  the  actual  origin  of  the  Ascidian  heart ; 
although  it  is  simply  a  dilated  tubular  structure,  yet  it 
arises  entirely  independently  of  and  prior  to  the  rest  of 
the  vascular  system  at  a  time,  in  fact,  before  the  formation 
of  the  muscular  mantle  and  before  the  atrial  cavity  has  so 
far  extended  itself  as  to  almost  entirely  replace  the  original 
body-cavity.  The  blood-sinuses  of  the  Ascidians  are  rem- 
nants of  the  latter. 

With  the  formation  and  growth  of  the  atrial  cavity,  the 
perforation  of  the  stigmata,  and  the  development  of  the 
muscular  mantle,  the  original  body-cavity  becomes  reduced 
to  a  system  of  narrow  canal-like  spaces  which  constitute 
the  above-mentioned  blood-sinuses.  The  oceneral  distribu- 
tion  of  the  blood-sinuses  can  be  made  out  from  Fig.  95. 
There  are  two  main  longitudinal  sinuses,  one  below  the 
endostyle  and  another  above  the  dorsal  lamina,  while 
others  are  scattered  irregularly  in  the  muscular  mantle ; 
others  again  lie  in  amongst  the  viscera  forming  the  inter- 
spaces between  the  various  parts ;  and  finally  the  bran- 
chial bars  between  the  stigmata  are  all  hollow,  and  their 
cavities  are  placed  in  communication  with  the  system  of 
sinuses  at  intervals  as  shown  in  Fig.  95. 

The  periodic  contraction  of  the  heart  of  Ascidians  takes 
place  on  a  highly  characteristic  and  unique  plan.  Each 
systole  occurs  as  a  peristaltic  wave  of  contraction  passing 


m 


.LVATOMY  AXD   DEVELOPMENT. 


193 


from  one  end  of  the  heart  to  the  other ;  but  the  chief 
peculiarity  in  connexion  with  it  is,  that  after  a  certain 
number  of  contractions  in  one  direction  the  heart  makes  a 
brief  pause  and  then  commences  to  contract  again  in  the 
opposite  direction,  and  so  it  goes  on  contracting  now  in  one 
direction  and  now  in  the  other.  This  phenomenon  of 
the  periodic  reversal  of  the  direction  of  contraction  of  the 
Tunicate  heart  is  known  as  the  recurrent  action  of  the 
heart,  and  was  discovered  in  1824  by  van  Hasselt. 
The  discovery  was  first  made  in  the  case  of  Salpa,  but  it 
has  since  been  found  to  hold  good  for  all  Tunicates. 

When  the  heart  contracts  from  its  posterior  to  its  an- 
terior extremity,  —  that  is  to  say,  in  the  postero-anterior 
direction, — the  blood  is  thereby  propelled  forwards  into  the 
blood-sinus  which  lies  below  the  endostyle,  and  from  this  it 
passes  into  sinuses  which  run  transversely  into  the  bran- 
chial bars.  In  the  basket-work  formed  by  the  intercross- 
ing of  the  branchial  bars,  the  blood  has  a  complicated 
and  irregular  course,  and  is  finally  collected  into  the  dorsal 
sinus  which  lies  above  the  dorsal  lamina.  Here  it  flows 
backwards,  and  after  passing  in  amongst  the  viscera  arrives 
back  to  the  heart.  (Other  branches  of  the  sinuses  pass 
into  the  test,  where  they  end  in  curious  knob-like  dilata- 
tions.) 

On  the  contrary,  when  the  heart  contracts  in  the  reversed 
or  antero-posterior  direction,  the  blood  which  has  already 
been  oxygenated  in  its  passage  through  the  branchial  bars 
is  sent  to  the  viscera  direct,  and  from  there  it  collects 
into  the  dorsal  sinus,  from  which  it  is  distributed  over  the 
branchial  sac,  and  so  into  the  sub-endostylar  or  ventral 
sinus,  in  which  it  flows  backwards  to  the  heart. 

On  account  of  the  above  peculiarities  relating  to  its 
independent  origin,  the  histological  structure  of  its  wall, 


i; 


Wf 


'm 


194 


T///-:  .ISC//)/. I. vs. 


I 


and  its  recurrent  action,  the  Tunicate  heart  would  appear 
to  be  a  unique  or<2jan  peculiar  to  the  group  of  the  Ascid- 
ians  and  analogous  but  not  homologous,  or  only  incom- 
pletely so,  with  the  heart  of  the  Vertebiates. 

Again,  the  vascular  system  of  an  Ascidian  is  only  func- 
tionally comparable  to  that  of  Amphioxus,  since  true  vessels 
provided  with  an  endothelial  lining  are  entirely  absent, 
their  place  being  taken  by  sinuses  which  arose  by  reduction 
from  the  original  body-cavity. 


Renal  Organs. 

The  renal  organs  of  the  Ascidians  have  no  apparent 
morphological  relation  to  those  of  Amphioxus,  and  therefore 
need  not  detain  us.  They  consist  of  a  group  of  bladder-like 
vesicles  with  cellular  walls  lying  around  the  intestine.  The 
products  of  excretion  (uric  acid,  etc.)  are  deposited  inside 
the  vesicles  in  the  form  of  solid  concretions.  There  is  no 
excretory  duct.  In  Molgii/a,  there  is  a  single  large  cylin- 
drical renal  sac  closed  at  both  ends  and  lying  on  the  right 
side  of  the  body,  behind  the  heart,  known  as  the  organ  of 
Bojanus. 

Comparison  bctiveen  an  .Ascidian  and  Amphioxus. 

Having  sketched  in  rough  outline  the  organisation  of  an 
adult  Ascidian,  we  are  now  in  a  position  to  consider  in 
what  respects  it  resembles  and  in  what  it  differs  from  that 
^f  Amphioxus.  We  shall  see  that  some  of  the  most  funda- 
mental differences  will  be  made  good  by  the  structure  of 
the  larva, —  such  as  the  absence  of  a  dorsal  nerve-tube  and 
of  a  notochord. 

Let  us  first  consider  the  resemblances  between  an  adult 
Ascidian  and  Amphioxus. 


MMa:: 


ANATOMY  AXD  DEVKI.OPMKXT. 


195 


of 


an 
in 

hat 

ida- 
of 

ind 


In  both  cases  the  pharynx  is  perforpted  by  a  great  num- 
ber of  ij^ill-apcrturcs  (gill-slits,  stigmata),  converting  it  into 
a  hmncliial  sac  and  opening  into  an  atrial  or  pcrihranchial 
cavity  instead  of  directly  to  the  exterior.  At  the  base  of 
the  pharynx  there  is  a  longitudinal  gland  consisting  of  a 
groove  open  throughout  its  whole  length  towards  the  cavity 
of  the  pharynx,  and  known  as  the  oidostylc,  whose  histo- 
logical character  is  closely  similar  in  the  t"*o  cases.  From 
the  anterior  extremity  of  the  endostyle  a  ciliated  band  of 
columnar  cells  passes  round  the  wall  of  the  pharynx  on 
each  side,  in  front  of  the  gill-openings,  and  abuts  on  the  dor- 
sal border  of  the  pharynx,  along  which  it  is  continued  back- 
wards in  connexion  with  the  dorsal  lamina  in  the  one  case 
and  the  liypcrpharyugeal  groove  in  the  other.  This  band 
forms  a  circlet  round  the  pharynx  behind  the  velum,  and  is 
the  peripharyngeal  band*  We  shall  find  also  that  the 
Ascidian  hypophysis  is  essentially  homologous  with  the 
olfactory  pit  of  Amphioxus. 

In  the  Ascidians  there  are  sphincter  muscles  round  the 
buccal  and  atrial  siphons,  and  inside  the  former,  in  front  of 
the  peripharyngeal  band  (pericoronal  groove),  there  is  a 
circlet  of  tentacles  corresponding  perhaps  to  the  velar 
tentacles  of  Amphioxus.     (Cf.  Fig.  94,  /;/.) 

The  differences  between  the  structure  of  an  adult  Ascid- 
ian and  of  Amphioxus  may  appear  to  outweigh  the  resem- 
blances, but  it  must  be  remembered  that  they  are  all 
correlated  with  and  accessory  to  the  one  great  difference 
in  the  mode  of  existence  of  the  respective  types. 

An  Ascidian  is  sessile  ;  Amphioxus  is  free.  The  former, 
as  it  were,  builds  its  house  upon  a  rock  and  is  immovable  ; 
the  latter  lives  in  the  shifting  sands,  and  is  capable  of 
extremely  active  locomotion. 

*  As  mentioned  above,  this  band  is  usually  grooved  in  the  Ascidians. 


'»!il 


)A 


'  !'ll*l  I 


196 


yy/A  AsciD/.txs. 


In  correlation  with  this  sessile  habit  of  existence  w  ;  find 
that  the  Ascidians,  in  contrast  to  Amphioxus,  are  hermaph- 
rodite,—  an  almost  universal  condition  among  sessile  orp;an- 
isms  of  every  description.  They  arc  unsegmented,  the 
muscles  not  being  divided  up  into  nfyotomes  ;  and  none  of 
their  organs  (gonads,  lenal  organs,  etc.)?re  metamerically 
repeated,  unless  we  regard  the  successive  transverse  rows 
of  stigmata  in  the  wall  of  the  branchial  sac  as  evidence  of 
metamerism.  It  is,  however,  of  a  totally  different  nature 
from  the  metamerism  of  the  gill-slits  of  Amphioxus,  and 
wc  shall  see  that  only  in  the  earlier  stages  of  their  devel- 
opment can  the  stigmata  of  the  Ascidians  be  compared 
with  the  former. 

Another  of  the  most  characteristic  accompaniments  of 
a  sessile  mode  of  life  is  the  U-shaped  alimentary  canal. 
Instead  of  being  a  straight  tube  with  a  posteriorly  directed 
anus  as  in  Amphioxus,  the  alimentary  canal  of  the  Ascid- 
ians is  doubled  up  upon  itself,  the  rectum  is  directed  for- 
wards, and  the  anus  opens  into  the  atrial  cavity.  The 
absence  of  a  dorsal  nerve-tube  and  notochord  in  the  adult 
Ascidian  has  been  indicated  above. 

In  spite  of  these  great  differences,  the  presence  of  the 
endostyle  and  the  perforated  wall  of  the  pharynx  in  the 
adult,  and  above  all  the  features  in  the  embryonic  and 
larval  development,  entitle  the  Ascidians  to  be  defined  as 
more  or  less  Aviphioxus-likc  creatures  which  have  become 
adapted  to  a  sessile  habit  of  existence. 


DEVELOPMENT  OF   ASCIDIANS. 


The  first  accurate  and  detailed  account  of  the  embryonic 
development  of  Ascidians  was  the  classical  memoir  pub- 
lished in  1867  by  Kowalevsky  in  the  Memoires  de 
I'Academie  imperiale  des  Sciences  de  St.  Petersbourg. 


AXAJ'OMY  AXJ)   nEVELOPMEXT. 


197 


The  Ascidian  larva  was  known  long  before  this  time, 
and  the  external  features  of  its  metamorphosis  were  de- 
scribed in  1838  jointly  by  Audouin  and  Milnk-Edwards, 
to  whom  the  discovery  of  the  free-swimming  larva  is  due. 
Furthermore,  the  internal  structure  of  the  tailed  larva, 
and  even  the  histological  structure  of  the  axial  rod  of  the 
tail,  was  described  with  some  accuracy  by  Kkohn  in  1852, 
but  in  ignorance  of  the  details  of  the  embryonic  devel- 
opment, he  was  unable  to  give  the  rght  morphological 
interpretation  to  the  various  parts,  and  did  not  identify 
the  axial  rod  with  the  notochord  of  the  hiirher  forms. 


eimft 


•yonic 
pub- 


Scgmcntatio)i  and  Gastnilatioti. 

The  segmentation  of  the  egg,  the  formation  of  a  hollow 
one-cell-layered  blastula,  and  the  flattening  and  subse- 
quent invagination  of  one  side  of  the  blastula  to  form 
the  two-cell-layered  gastrula,  take  place  on  a  plan  so 
essentially  similar  to  what  has  been  described  above  for 
Amphioxus  that  it  is  not  necessary  to  dwell  at  length 
upon  them  here.  Suffice  it  to  point  out  that  the  segmen- 
tation of  the  Ascidian  egg  takes  place  typically,  according 
to  VAN  Beneden  and  Julin,  on  a  strictly  bilateral  plan. 
That  is  to  say,  when  the  ovum  has  divided  into  two 
blastomeres,  right  and  left,  each  blastomere  represents 
and  will  give  rise  to  the  corresponding  half  of  the  larval 
body,  and  the  descendants  of  the  first  two  blastomeres 
can  be  distinguished  for  a  remarkably  long  time  on  each 
side  of  the  middle  line  of  the  embryo,  —  a  fact  which  is 
highly  characteristic  of  Ascidian  development. 

After  the  gastrula  has  begun  to  elongate,  and  the  blas- 
topore has  been  narrowed  down  by  the  approximation  of 
its  lips  to  a  small  aperture  situated  at  the  posterior  dorsal 
extremity  of  the  embryo,  the  formation  of  the  medullary 
plate  occurs. 


198 


THE  ASCWI.tXS. 


Formation  of  Medullary   Tube  and  Notochord. 

Here,  as  in  Amphioxus,  the  dorsal  wall  of  the  embryo 
flattens,  while  the  ventral  remains  convex,  and  the  ecto- 
dermic  cells  on  the  tlorsal  side  become  marked  off  from 
the  rest  by  their  larger  size  and  columnar  shape.  The 
medullary  plate  extends  nearly  to  the  front  end  of  the 
embryo,  while  posteriorly  its  cells  form  a  ring  round 
the  blastopore. 

In  the  formation  of  the  medullary  tube,  however,  there 
is  an  important  difference,  and  the  Ascidian  embryo  con- 
forms in  this  point  more  to  the  mode   of   development 


Fig.  98.  —  Transverse  sections  through  embryo  of  Clavelina  Rissoana,  to  show 
mode  of  formation  of  medullary  tube  and  mesoderm.     (After  Davidokk.) 

A.   Through  anterior  region  of  embryo,  with  medullary  groove  still  open. 

n,  Throu<;h  posterior  region,  with  closed  medullary  tube. 

c/i.  Rudiment  of  notochord.  ec.  Ectoderm,  en,  Endoderm.  mes.  Mesoderm. 
ni.g.  Medullary  groove,     tnj.  Medullary  tube. 

which  is  typical  of  the  higher  Vertebrates  than  does 
Amphioxus.  In  the  latter  the  medullary  plate  sinks 
bodily  below  the  level  of  the  surrounding  ectoderm,  which 
then  grows  over  it.  Subsequently  while  underneath  the 
ectoderm  the  medullary  plate  assumes  the  form  of  a 
half-canal  open  towards  the  ectoderm,  and  eventually  its 
margins  come  together  and  so  form  a  complete  tube. 

In  the  Ascidian  embryo  the  overgrowth  of  the  surround- 
ing ectoderm  and  the  folding  up  of  the  margins  of  the 


% 


.i.v.r/o.uy  WD  itEVEi.or.Mi-.XT, 


•99 


bryo 


'*C 


medullary  pUite  occur  simultaneously,  so  that  when  the 
latter  has  the  form  of  a  half-canal  it  is  not  closed  over 
by  a  layer  of  ectoderm,  but  is  open  to  the  exterior 
(Fi-  98). 

At  a  somewhat  later  staj^e  the  two  mcditllixry  folds  meet 
to<;ether  and  fuse  in  the  middle  line  (^'i^^  98  />),  and  this, 
combined  with  a  slight  forward  jjjrowth  of  the  posterior 
lip  of  the  blastopore,  leads  to  the  inclusion  of  the  latter 
in  the  medullary  tube, 
so  that  we  arrive  at  the 
condition  already  de- 
scribed for  Amphioxus, 
in  which  the  nerve-tube 
opens  in  front  to  the 
exterior  by  the  nciiropoir 
and  behind  into  the  ar- 
chenteron  by  the  blasto-       „.  .   ,.  ,        c  ..,  ,, 

Fig.  99.  —  .  /.   I'.nibryo  of  PhallNsia  mam- 
pore,      which      has     now    millntii  seen  in  optical  section  from  above,  to 

b,     ,     .    ,       show  notochorcl. 
ecome     converted     mtO  /,,   Section   through   tail   of  older  embryo 

the  ilVlD'CHtCVic  CailClL  °^  P/iallusia  mammilliita.     (After  KOWALEV- 


,en 

^M^ 

SKY.) 


Meanwhile     the      cells  ch.  XotochorU.    ec.  Ectoderm,    eii.  Kndo- 

r  ■  .  1  1  1  11    derm.     mes.  Mesoderm,    nt.  Medullary  tube. 

lormmg  the  dorsal  wall 

of  the  archenteron  in  its  posterior  two-thirds  begin  to 
gather  themselves  together  to  form  the  notochord  (Figs. 
98  and  99).  The  cells  forming  the  notochord  are  at  first 
arranged  end  to  end  (Fig.  99),  and  subsequently  interlace 
in  the  manner  described  above  for  Amphioxus. 


Origin  of  Mesoderm. 

At  about  the  same  time  in  which  the  formation  of  the 
medullary  tube  and  notochord  is  going  on,  the  mesoderm 
begins  to  put  in  its  appearance,  and  this  is  the  first  event 
in   the  development  in  which  there  is  an  important  dif- 


/^ 


»jr 


200 


THE  ASCIDIAXS. 


'■') 


I'j, 


ference  between  the  Ascidian  and  Amphioxus.  The 
mesoderm  in  the  Ascidian  embryo  does  not  arise  as  a 
series  of  archenteric  pouches,  but  is  produced  on  each  side 
by  a  solid  proliferation  of  cells  from  the  primitive  endoderm 
which  lines  the  archenteric  cavity.     This  solid  proliferation 

begins  in  the  middle  region 
of  the  embryo  near  the  an- 
terior limit  of  the  notochord, 
and  extends  backwards  (Figs. 
98  and  100).  It  takes  place 
from  the  dorso-lateral  cells 
of  the  endoderm,  in  a  posi- 
tion corresnonding  to  that 
at  which  the  mesoblastic 
pouches  of  Amphioxus  grow 
out  from  the  archenteron. 
The  mesoderm  of  the  As- 
cidian embryo  therefore 
Fig.  100.  — Embryo  of  <:/ai'.f//>a/?/j-  agrees  with  that  of  the  em- 

soana  seen  from  above,  to  show  'he  re- 
lation of  parts.    (Simplified  after  VAN  bryo  of  Amphioxus  m  being 
BENEDKNandjuuN.)  dcrivcd  from  the  primitive 

tip.  Neuropore.    en.  Endoderm.   ent.c,  " 

Enteric  cavity.  m.t.  Medullary  tube,  endodcrm,  but  differs  in  bc- 
mes.  Mesodermic  band,    ch,  Notochord.    .  i-  i         •. 

ec.  Ectoderm.  mg  solid  and  unsegmcntcd.* 

*  For  a  recent  and  elaborate  discussion  of  the  origin  of  the  mesoderm  in 
the  Ascidians  see  \'i)N  Davidokf's  Untersuchttngen  zttr  Ent-wicklungsgeschichte 
der  Distaplia  niagitihirTHj,  etc.,  II.  Allgemeine  Entiv,  der  KeimhliUter.  Mitth. 
Zool.  Stat.  Neapel,  IX.     1891.     pp.  533-651. 

As  shown  by  van  Beneden  and  Julin  in  Clu'ielina,  the  primary  meiioderm 
of  the  .'\scidian  emljryo  can  be  detected  at  a  much  earlier  stage  of  development 
than  in  Amphioxus. 

I  have  studied  the  origin  of  the  mesoderm  in  Cynthia  papulosa  and  found 
that  the  primary  mesoderm  cells  are  to  be  distinguished,  by  their  poverty  in 
food-yolk,  from  the  remaining  endoderm,  at  the  commencement  of  gastrula- 
tion  (at  the  so-called  plakula-stage).  They  occur  in  the  form  of  a  crescent 
round  the  posterior  margin  of  the  blastopore,  and  are  carried  in  by  the  invagi- 
nation, and  then  increase  in  number  by  mitotic  division.     In  Cynthia,  these 


ANATOMY  AND  DEVEL    PMENT. 


20 1 


We  thus  have  two  solid  longitudinal  mcsodermic  bands 
inserted  between  the  ectoderm  and  endoderm.  Anteriorly 
the  mesodermic  bands  consist  of  several  layers  of  cells  super- 
imposed one  above  the  other  (Fig.  98),  but  farther  back 
they  consist  of  only  one  layer  of  cells.  Both  portions  of 
the  mesoderm — namely,  the  anterior  two-  or  three-layered 
and  the  posterior  one-layered  portions — arise  in  continuity 
with  one  another,  but  they  have  different  fates,  the  former 
eventually  breaking  up  into  loose  cells  which  float  about 
in  the  body-cavity  and  constitute  the  so-called  vicscnchynic, 
the  latter,  on  the  other  hand,  becoming  converted  into  the 
musculature  of  the  tail ;  whence  the  former  is  spoken  of 
as  the  gastral  and  the  latter  as  the  caudal  mcsoderut. 

Outgrowth  of  Tail. 

In  Amphioxus,  at  the  stage  corresponding  to  that  of 
which  we  have  been  speaking  —  namely,  when  the  embryo 
has  an  oval  or  sub-elliptical  shape  —  it  bursts  through  the 
vitelline  membrane  inside  which  it  has  air  jady  been  rotat- 
ing for  some  time  by  means  of  the  cilia  of  the  ectoderm, 
and  escapes  into  the  open  sea.  This  is  not  the  case, 
however,  with  the  Ascidian  embryo.  The  latter  is  never 
ciliat'  d  externally,  and  it  remains  enclosed  within  the  fol- 
licular membrane  throughout  the  whole  of  the  cnibiyonic 
period  of  development. 

After  the  stage  in  question,  the  growth  in  the  length  of 
the  embryo  is  accompanied  by  a  ventral  cnn'aturc,  owing 
to  the  confined  space  in  which  it  is  contained.  Moreover, 
the  increase  in  length  is  not  due  to  a  simple  elongation  of 
the  entire  body  of  the  embryo,  as  is  the  case  with  Amphi- 

primary  mesoderm  cells  appear  to  give  rise  almost  exclusively  to  the  caudal 
mesoderm,  while  the  gastral  mesoderm  appears  to  l^c  added  in  front  by  prolifera- 
tion .'rom  the  primitive  endoderm  as  described  above. 


/'iii 


w 

ipr 

1 

1 

i 

»'. 

' 

202 


7V/E  ASCID/AA'S. 


''i  i 


\\ 


"*  .    fi- 


oxus,  but  it  is  merely  due  to  the  outgrowth  of  the  tail  from 
the  body  of  the  embryo  (Fig.  loi). 

The  structures  involved  in  the  outgrowing  tail  are  the 
dorsal  nerve-tube,  the  notochord,  the  caudal  mesoderm, 
which  lies  on  each  side  of  the  notochord,  and  will  give  rise 
to  the  muscles  of  the  tail,  and  finally  a  solid  cord  of  endo- 
derm  consisting  of  two  rows  of  cells  placed  side  by  side 

below  the  notochord  (Fig. 
99  B).  As  soon  as  the  tail 
begins  to  grow  out,  the  neu- 
renteric  canal  becomes  ob- 
literated, and  shortly  after- 
wards the  anterior  neuropore 


n.t 


-ec 


Fig.  loi.  —  Embryo   of  Phaiiiisia  closcs  up  temporarily.    At  a 

mammilLita  in  side  view,  to  show  com- 
mencing    outgrowth     of     tail.       (Aftc 

KOWAI.EVSKY.) 

ch.  Notochord.   ec.  Ectoderm,  en.  En- 
doderm.    mes.  Mesoderm ;   the  cells  in- 
dicated by  dark  outlines,  beneath  which    ,  i    4.    u 
may  be  seen  the  notochord  and   caudal    DUCCai    tUDe. 
endoderm.    n.p.  Neuropore.    «./.  Medul- 
lary tube. 


later  period,  as  we  shall  see, 
it  reopens  ;  not,  however,  to 
the   exterior,    but    into   the 


As  the  tail  grows  in 
length,  it  becomes  coiled 
round  about  the  body  of  the  embryo,  attaining  two  or 
three  times  the  length  of  the  latter. 

The  cord  of  endoderm  cells  in  the  tail  of  the  Ascidian 
larva  has  been  supposed  to  represent  a  rudimentary  intes- 
tine homologous  with  the  straight  intestine  of  Amphioxus, 
the  larval  tail  being  on  this  view  equivalent  to  the 
post-branchial  portion  of  the  trunk  in  Amphioxus  This 
view,  however,  is  probably  not  correct,  although  there  is 
something  to  be  said  in  favour  of  it.  The  probability  is 
that  the  tail  of  the  Ascidian  larva  or  tadpole,  as  it  is  often 
called,  is  an  organ  which  has  been  specially  elaborated  in 
the  course  of  its  evolution  for  the  particular  benefit  of  the 
Ascidians,  since  (exclusive  of  the  pelagic  forms)  it  is  their 


^INATOMY  AXD  DLVELOVMEiXT. 


203 


sole  organ  of  locomotion,  and  hence  of  transportation  from 
place  to  place ;  thi  only  being  possible  during  the  larval 
period. 

As  a  rule,  the  larval  phase  of  an  Ascidian's  existence  is 
a  remarkably  brief  one,  and  there  is  on  this  account  all  the 
more  need  for  an  effective  propelling  organ,  which  will 
enable  the  larva  to  arrive  at  a  suitable  resting-place. 

In  Amphioxus,  as  described  above,  locomotion  is  ef- 
fected by  serpentine  movements  of  the  whole  trunk  in 
virtue  of  its  muscle-segments,  and  there  is  therefore  no 
need  for  a  tail  in  addition  ;  but  there  is,  nevertheless,  a 
short  post-anal  extension  of  the  body,  which  alone  can  be 
regarded  as  the  homologue  of  the  tail  of  the  Ascidian  larva. 
In  the  latter  (e.g.  Ciona,  Phallusia,  etc.)  the  muscles  are 
entirely  confined  to  the  tail,  none  being  formed  in  the  body 
proper,  until  after  the  resorption  of  its  caudal  appendage. 

On  the  view  which  I  am  endeavouring  to  make  clear, 
it  follows  that  the  tail  of  the  Tunicate  tadpole  is  of  the 
same  nature  as  that  of  the  Amphibian  tadpole,  and,  in  fact, 
of  the  craniate  Vertebrates  generally,  and,  as  has  just  been 
said,  is  only  represented  by  the  short  post-anal  section  of 
the  trunk  in  Amphioxus. 

The  solid  cord  of  endoderm  in  the  tail  is  not,  therefore, 
a  rudiment  of  a  primitive  intestine,  but  it  is  analogous  to, 
even  if  not,  as  first  suggested  by  Balfour,  homologous 
with,  the  so-called  post-anal  gut  which  occurs  in  the  em- 
bryos of  the  higher  Vertebrates,  and  bears  a  similar  rela- 
tion to  the  formation  of  the  tail  that  the  endoderm-cord  in 
the  Ascidian  embryo  does. 

Thus  in  the  typical  Ascidian  embryo  the  elongation  of 
the  trunk  (body  proper)  does  not  take  place  to  any  consid- 
erable extent  during  the  embryonic  or  oven  larval  period, 
but  only  after  the  metamorphosis. 


'■h 


w 


■i 


204 


7V/E  ASCIDIANS. 


«r  H^ 


S'U  ^iin 


if  if 


With  the  formation  of  the  tail  the  enteric  cavity  be- 
comes confined  as  a  closed  sac  to  the  anterior  portion  of 
the  embryo.  It  is  bounded  dorsally  by  the  nerve-tube, 
which  is  somewhat  dilated  in  this  region,  and  in  front,  at 
the  sides  and  below,  it  is  in  close  contiguity  with  the 
ectoderm. 

Formation  of  the  Adhesive  Papilla: . 

At  a  much  later  stage  than  that  represented  in  Fig.  loi, 
the  ectoderm  bounding  the  conver  anterior  extremit;^  of 
the  body  becomes  raised  up  into  three  prominences,  whose 
relations  to  one  another  are  those  of  the  corners  c^  a  tri- 
angle. They  are  due  to  the  ectodermic  cells  at  the  respec- 
tive points  assuming  a  high  columnar  shape.  They  become 
eventually  raised  very  much  above  the  adjoining  surface  of 
the  ectoderm,  and  become  the  adhesive  papillcB  or  fixing 
glands  of  the  larva.  The  cells  composing  them  acquire  the 
power  of  secreting  a  viscid  substance,  by  which  the  larva 
can  fix  itself  to  any  favourable  surface  (Fig.  102). 

Cerebral  Vesicle  and  its  Sense-organs. 

We  have  spoken  above  of  the  dilated  anterior  portion  of 
the  nerve-tube.  This  is  the  part  of  the  central  nervous 
system  which  undergoes  the  most  striking  subsequent 
changes.  By  a  gradual  widening  of  its  cavity,  accom- 
paniv,d  by  a  local  thinning  cut  of  its  wall,  this  portion 
of  the  neural  tube  lying  in  front  of  the  notochord  becomes 
transformed  into  a  spacious  sub-spherical  vesicle,  known 
as  the  cerebral  vesiele  {Y\g.  102). 

While  the  anterior  portion  of  the  neural  tube  is  enlarg- 
ing to  form  the  cerebral  vesicle,  granules  of  black  pigment 
are  deposited  by  certain  cells  in  the  dorsal  wall  of  the 
vesi  le.     The  granules  are  at  first  scattered  about  in  the 


||; 


ANA  TOM  1  •  AND  DE I  'EL  Ol'MENT. 


205 


interior  of  the  cells.  The  most  anterior  of  the  cells  con- 
taining the  pigment  is  at  first  distinguished  from  the 
others  solely  on  account 
of  the  fajt  that  the  pig- 
ment-granules which  it 
contains  are  somewhat 
larger  than  those  in  the 
succeeding  cells.  (Cf. 
Fig.  103.) 

Later      on,       however,  pig.    j^^.  -  Embryo   of   Avidia   mentula 

the   first   niffmented   cell  s'i"''"v  before   hatching;   from  the  right  side. 

^    *»                  .  (After  Wil.l.KY.) 

is  seen  to  separate  itself  ch.   NotochorU,   undergoing    vacuohsation. 

c             .1           .1                     1    -i  '•    Eye.      ent.c.   Enteric    cavity,      f.    Adhesive 

from    the    others,    and    it  ^^^^^^^     „,,    interior   portion   of  nerve-tubc 

becomes  gradually  trans-    (spinal  cord),    o.  Otocyst.  lying   on  the  floor 

of    the    cerebral    vesicle    and    projecting    up 
ferred   by    a    differential    freely  into  its  cavity.    r.a.  Right  atrial  involu- 
i-i-        r    i-i.  11        r    tion.    st.  Stomodceum. 

growth    of   the  wall    of 

the  vesicle  down  the  right  wall  to  its  final  position  in  the 
ventral  wall  of  the  vesicle  (Figs.  102,  103).  This  cell  is 
the  otocyst,  and  the  pigment-granules  become  consolidated 
together   to  form   the  otolith.     The   latter   is   apparently 


Fig.  103.  —  Optical   sections  through   cerebral  vesicle  of  embryos  of  Ascidia 
vieutula,  to  show  mode  of  origin  of  eye  and  otocyst.     (After  WiLl.KY.) 
e.  Eye.     o.  Otocyst. 

extruded  from  the  cell  (otocyst)  in  which  it  was  originally 
formed,  and  the  latter  assumes  a  cup-shape,  in  the  hollow 
of  which  the  otolith  lies.  The  two  structures  together 
form  the  so-called  auditory  organ,  whose  function  may  be 
not  so  much  of  an  auditory  nature  as  that  of  an  equilibrat- 
ing apparatus. 


/',Jf 


Pipp 


206 


THE  ASCIDIANS. 


I  * 


The  other  pigment-cells  of  the  dorsal  wall  of  the  cerebral 
vesicle  collect  themselves  together  and  form  a  slight  pro- 
tuberance in  the  right  dorso-lateral  corner  of  the  vesicle, 
while  the  pigment-granules,  which  were  at  first  scattered 
about  in  the  interior  of  the  cells,  become  concentrated  at 
their  converging  extremities  towards  the  cavity  of  the 
vesicle.  And  in  this  way  is  formed  the  single  eye  of  the 
Ascidian  tadpole ;  the  original  pigment-producing  cells 
constitute  the  trtina,  which  retains  its  primitive  position 
as  part  of  the  epithelial  wall  of  the  brain.* 

Subsequently  two  or  three  cells  from  the  adjoining  wall 
of  the  vesicle  take  up  a  position,  one  above  the  other,  in 
front  of  the  mass  of  pigment  and,  having  previously,  by 
an  alteration  in  the  character  of  their  protoplasmic  con- 
tents, acquired  a  high  refractive  index,  constitute  the  lens 
of  the  eye,  which  projects  obliquely  downwards  into  the 
cavity  of  the  vesicle.     (Cf.  Fig,  105  A.) 

The  cerebral  vesicle  of  the  Ascidian  tadpole  is  the  un- 
doubted homologue  of  the  corresponding,  but  less  pro- 
nounced, structure  in  Amphioxus.  It  differs  from  the 
latter  in  lying  wholly  in  front  of  the  anterior  extremity  of 
the  notochord,  in  possessing  a  more  highly  organised  eye, 
provided  with  a  cellular  lens,  and  in  the  presence  of  an 
otocyst,  which,  as  we  have  seen,  is  evolved  from  the  same 
group  of  cells  which  gave  rise  to  the  eye. 

The  eye  of  the  Tunicate  tadpole  agrees  fundamentally 
with  the  type  of  eye  peculiar  to  the  Vertebrates,  in  that 
the  retina  is  derived  from  the  wall  of  the  brain.     On  this 

*  The  fact  that  the  lens  of  the  Tunicate  eye  as  well  as  the  retina  and 
the  otocyst  arise  by  differentiation  of  one  and  the  same  epithelial  layer  of 
the  primitive  cerebral  vesicle,  has  recently  been  described  by  Sai.knskv  for 
the  larva  of  Distaplia,  magnilarva.  (W.  Sai.knskv.  AForphologische  Studien 
an  Tunicaten  :  I.  Ucber  das  Nervensysteni  der  Larvcn  it.  Emhryonen  von 
Distaplia  masnilarva.     Morph.  Jahrb.  XX.     1S93.     PP- 4S-74O 


.IX.l  TOM \ ■  . IXl)  DE  I  EL OI'MEXT. 


:o7 


account  it  is  called  a  myclonic  eye.  In  the  typical  Inverte- 
brate eye,  on  the  contrary,  the  retinal  cells  are  differen- 
tiated from  the  external  ectoderm. 


I  and 
ir  of 

for 
\iien 

VOK 


Couiparisoii  of  Tiinieate  Eye  ivith  the  Pineal  Eye. 

The  Tunicate  eye,  however,  differs  essentially  from  the 
paired  eyes  of  the  craniate  Vertebrates  in  that  the  lens,  as 
well  as  the  retina,  is  derived  from  the  wall  of  the  brain. 
The  lens  of  the  lateral  eye  of  the  Vertebrates  is  derived 
by  an  invagination  of  the  external  ectoderm,  which  meets 
and  fits  In  with  the  retinal  cup  at  the  end  of  the  optic 
vesicle. 

It  is,  therefore,  an  extremely  interesting  fact  which  was 
pointed  out  by  Baldwin  Spencer,  that  the  Tunicate  eye 
agrees,  in  respect  of  the  origin  of  its  lens,  with  the  parietal 
or  pineal  eye  of  the  Lacertilia,  in  which  the  lens  is  likewise 
derived  from  cells  wliich  form  part  of  the  wall  of  the 
cerebral  outgrowth  which  gives  rise  to  the  pineal  body. 

The  pineal  body  is  another  of  those  remarkable  rudi- 
mentary structures  whose  constant  presence  in  all  groups 
of  Vertebrates  forms  such  an  eminently  characteristic 
feature  of  their  organisation.  It  develops  as  a  hollow 
median  outgrowth  from  the  dorsal  wall  of  the  brain 
(thalamencephalon),  the  distal  extremity  of  which  dilates 
into  a  vesicle  and  becomes  separated  from  the  proximal 
portion.* 

For  a  long  time  the  pineal  body  was  a  persistent  enigma 

*  According  to  the  most  recent  work  on  the  sul>ject  the  distal  vesicle  be- 
comes entirely  constricted  off  from  the  primary  epiphysial  (pineal)  outgrowth 
of  the  brain,  and  the  parietal  nerve  does  not  represent  the  primitive  connex- 
ion of  the  pineal  eye  with  the  roof  of  the  brain,  but  it  arises  quite  inde- 
pendently of  the  proximal  portion  of  the  epiphysis. 

See  A.  Klinckowstki'jM,  Bi'ttra^e  ziir  A'cniifniss  i/i's  Pariefiilaiiges.. 
Zoologische  JahrbUcher  (Anat.  Abth.),  VII.     1S93.     pp.  249-280, 


208 


THE   ASCID/AAS. 


:S 


: 


LL&^i:  ^^1 


and  the  subject  of  much  speculation,  one  of  the  most  cele- 
brated hypotheses  with  regard  to  its  significance  being 
that  of  Descartes,  who  regarded  it  as  the  seat  of  the 
soul. 

More  recently  it  has  been  shown  to  represent  a  rudi- 
mentary, unpaired  eye.  Although  in  most  cases,  curiously 
enough,  it  exhibits  in  existing  forms  no  trace  of  an  eye- 
structure,  it  has  been  shown  by  de  Graaf  and  Spencer 
that,  as  a  matter  of  fact,  in  many  lizards  the  distal  vesicle 
does  actually  become  converted  into  an  eye  which,  though 
of  a  rudimentary  character,  is  possessed  of  a  retina,  pig- 
ment, and  lens.  In  these  forms  the  pineal  body  pierces 
the  roof  of  the  cranium,  occasioning  the  parietal  foramen, 
which  is  so  characteristic  of  the  Lacertilian  skull,  and  the 
pineal  eye  lies  outside  the  cranium  immediately  below  the 
skin,  through  which  it  can  be  distinguished  in  external 
view  by  the  presence  of  a  modified  scale  placed  above  it. 

In  the  animals  below  the  lizards  in  the  scale  of  organi- 
sation (Amphibians  and  Fishes),  as  well  as  in  those  above 
them,  the  distal  vesicle  of  the  pineal  body  apparently  does 
not  become  so  far  differentiated  as  to  be  recognised  as 
an  actual  eye,  except  in  the  case  of  the  Cyclostome  fishes, 
where,  as  shown  by  Beard,  it  presents  the  three  essential 
elements  of  an  eye  ;  namely,  retina,  pigment,  and  lens, 
lying,  however,  inside  the  cartilaginous  cranium. 

The  facts  in  our  possession  would  seem  to  indicate 
that  the  remote  ancestors  of  the  Vertebrates  possessed 
a  median,  unpaired,  myelonic  eye,  which  was  subsequently 
replaced  in  function  by  the  evolution  of  the  paired  eyes. 
It  would,  however,  be  premature  either  to  assert  this  or  to 
express  it  as  a  definite  opinion,  especially  since,  in  refer- 
ring to  the  evolution  of  the  paired  eyes  of  Vertebrates, 
we  are  bordering  on  ground  upon  which  I  have  no  imme- 


ANATOMY  AXD  DEVELOPMENT. 


209 


diate  intention  of  treading.  The  pineal  eye  may  not  have 
been  primitively  so  much  an  organ  of  vision  as  a  light- 
perceiving  organ,  as  is  no  doubt  the  case  with  the  eye  of 
the  Tunicate  tadpole. 

We  may  at  least  conclude  that  there  can  be  no  doubt 
that  the  Tunicate  eye  is  the  functional  homologue  of  the 
pineal  eye  of  the  higher  Vertebrates,  as  Spencer  sug- 
gested. 

Stomodceal  and  Atrial  Involutions. 

By  the  time  that  the  cerebral  vesicle  of  the  Ascidian 
embryo  with  its  contained  sense-organs  (eye  and  otocyst) 
is  approaching  the  completion  of  its  full  development,  no 
less  than  three  ectodermic  invaginations  occur  in  the  body 
of  the  embryo.  One  of  these  is  situated  immediately  in 
front  of  and  in  contact  with  the  anterior  wall  of  the  cere- 
bral vesicle,  the  blind  end  of  the  involution  pressing 
against  the  subjacent  endoderm.  This  is  the  stoniodceum, 
and  its  formation  is  preliminary  to  the  perforation  of  the 
mouth  which  takes  place  later,  and  places  the  stomodoeum 
in  open  communication  with  the  portion  of  the  enteric 
cavity  ^"hich  will  become  the  branchial  sac  (Fig.  102).  It 
should  be  emphatically  noted  that  the  stomodoeal  invagi- 
nation occurs  in  the  dorsal  middle  line  immediately  adja- 
cent to  the  anterior  extremity  of  the  central  nervous 
system. 

The  other  two  ectodermic  invaginations  occur  symmetri- 
cally, one  to  the  right  and  the  other  to  the  left  of  the 
dorsal  middle  line,  behind  the  region  of  the  cerebral  vesicle, 
and  constitute  the  pair  of  atrial  involutions,  which,  by  their 
subsequent  growth  and  modification,  give  rise  to  the  atrial 
or  peribranchial  cavity.  We  see,  therefore,  that  the  epi- 
thelium which  forms  the  lining  membrane  of  this  cavity 
is,  a:  in  Amphioxus,  derived  from  the  external  ectoderm. 


i.^ 


'IIP 


P« 


210 


THE   ASCID/AXS. 


li  . 


For  some  considerable  time  after  the  metamorphosis  the 
young  Ascidian  possesses  two  separate  atrial  cavities,  right 
and  left,  each  opening  to  the  exterior  by  its  own  atrial 
aperture.  Eventually  the  two  cavities  extend  round  the 
branchial  sac  dorsally,  so  that  their  walls  come  into  contact 
in  the  dorsal  middle  line,  and  finally  the  dividing  line 
breaks  down,  and  they  become  continuous  one  with  another 
dorsally,  remaining  separated  ventrally,  as  described  above. 

At  the  same  time  that  the  two  atrial  cavities  grow 
towards  one  another^  their  external  apertures  b'jcome  in- 
volved in  the  same  process  of  growth,  and,  moving  together, 
finally  fuse  in  the  dorsal  middle  line,  and  so  form  the  single 
atrial  or  cloacal  aperture  of  the  adult.* 

Beyond  agreeing  in  its  ectodermal  origin,  there  might 
appear  to  be  not  much  in  common  between  the  mode  of 
development  of  the  atrial  cavity  in  the  Ascidians  and  in 
Amphioxus. 

No  morphologist  would  recognise  a  fundamental  differ- 
ence in  the  fact  that  the  right  and  left  halves  of  the  atrial 
cavity  in  Amphioxus  arise  by  a  single  median  involution  of 
the  ectoderm,  instead  of  from  a  pair  of  involutions,  and  that 
they  are  from  the  first  continuous  with  one  another  instead 
of  becoming  so  secondarily  (Fig.  104). 

In  like  manner,  the  fact  that  the  two  halves  of  the  atrial 
cavity  are  continuous  with  one  another  ventrally  in  Amphi- 
oxus and  dorsally  in  the  Ascidians,  is  easily  brought  into 
correlation  with  the  other  differences  in  the  organisation 
of  the  two  types,  which  have  been  described  above,  and  is 
no  bar  to  our  regarding  the  atrial  cavity  of  the  one  as  being 
homologous  with  that  of  the  other. 

*  The  time  at  which  the  atrial  cavities  fuse  together  varies  very  much  in 
different  genera.  In  Molgula  manhattensis,  for  instance,  whose  stigmata 
develop  on  a  similar  plan  to  those  of  Ciona  (see  below),  there  is  a  single 
atrial  aperture  at  the  moment  of  the  metamorphosis. 


ANA  TOM  V  AXD  DE I V.  /.  OPMEXT. 


211 


One  feature  in  connexion  with  the  formation  of  the 
atrial  cavity,  in  which  the  Ascidians  stand  in  marked 
contrast  to  Amphioxus,  does,  however,  require  a  special 
explanation. 

Whereas  in  Amphioxus  the  atrial  involution  has  the  form 
of  a  longitudinal  groove,  in  the  Ascidians  it  occurs  on  each 
side,  as  a  local  inpushing  of  the  ectoderm  with  a  minute 
circular  orifice  of  invagination.'^ 

The  fact  has  already  been  stated  above  that  the  elonga- 
tion of  the  body  proper  of  an  Ascidian  embryo  or  larva  does 
not,  in  the  main,  take  place  until  after  the  metamorphosis. 


A  li 

Fig.  104.  —  Diagrammatic  transverse  sections,  to  illustrate  the  mode  of  forma- 
tion of  the  atrium  in  {A)  an  Ascidian  and  (Z/)  Amphioxus.     (After  WlLLEY.) 


being 


The  atrial  involutions  occur  at  a  time  when  the  tail  is 
rapidly  increasing  its  length ;  the  body  proper,  on  the  con- 
trary, remaining  stationary  so  far  as  increase  in  size  is 
concerned,  and  retaining  at  this  stage  approximately  the 
dimensions  which  it  possessed  when  the  tail  first  began  to 
grow  out.  Moreover,  they  occur  before  the  appearance  of 
any  gill-clefts  in  the  wall  of  the  branchial  sac,  so  that  in  the 
Ascidians  the  gill-slits  never  open  directly  to  the  exterior. 

In  Amphioxus,  on  the  other  hand,  there  is  no  such  delay 
in  the  elongation  of  the  body  of  the  embryo,  but  it  goes  on 
continuously  till  the  full  complement  of  myotomes  has  been 


i.2^ 


iftiffnr 


212 


'11  N:    ASCIDIAXS. 


vm 


W^  \  \ 


il 


formed.  The  post-anal  portion  of  the  body,  which  \vc  sup- 
pose to  be  the  homologue  of  the  tail  of  the  As.cidian  tad- 
pole, does  not  appear  until  a  somewhat  late  period  in  the 
development.  There  is  very  little  of  it  present  in  the  larva 
with  three  gill-slits  (Fig.  Tl). 

The  reason  of  this,  as  explained  above,  is  that  the  post- 
anal section  of  the  trunk  is  of  only  minor  functional  sig- 
nificance in  Amphioxus,  but  is  all-important  to  the 
Ascidian  larva,  and  consequently,  as  is  the  case  with 
many  other  structures  of  great  functional  importance  in 
the  various  groups  of  the  animil  kingdom,  it  exhibits  a 
precocious  development. 

Not  only,  therefore,  has  the  elongation  of  the  body  of 
Amphioxus  already  taken  place  before  the  occurrence  of 
the  atrial  involution,  but  the  primary  gill-slits  have  also 
broken  through  the  wall  of  the  pharynx,  and  open  freely  to 
the  exterior  before  the  atrium  begins  to  be  closed  in. 

In  Amphioxus,  then,  the  atrial  involution  has  been  drawn 
out  into  the  form  of  a  longitudinal  groove  because  it 
occurs  subsequently  to  the  elongation  of  the  body  and 
the  perforation  of  the  gill-slits. 

In  the  Ascidian  embryo  the  (paired)  atrial  involution 
has  the  form  of  a  simple  pit  with  a  circular  margin,  be- 
cause it  arises  before  the  elongation  of  the  body  proper 
of  the  embryo  and  before  the  perforation  of  the  gill-clefts, 
so  that  no  influence  has  been  at  work  to  draw  it  out  into 
the  form  of  a  groove. 

We  see,  therefore,  that  a  great  many  of  the  differences 
between  the  Ascidian  tadpole  and  the  larva  of  Amphi- 
oxus can  be  explained  sufficiently  to  allow  of  their  being 
brought  into  genetic  relation  with  one  another,  by  consid- 
ering the  relative  time  at  which  corresponding  develop- 
mental processes  take  place  in  the  two  cases. 


■«*>ai8»i! 


^i.v.ijo.vy  Axn  nEVELoiwiExr. 


^'3 


The   followin^^   tabic    will    help   tt)   make    this    matter 
clearer. 


Order 

OK 

ASCIDIAN. 

AMI'flKJXfS. 

OCCI'KRKNCK. 

1. 

(lastrulaticn. 

tiastrulation. 

9. 

Oval  fml)rv(j  with  medullary 

( )val    cmliryo  with    medullary 

tube,     neurL-ntcric     canal, 

tui>u,      ncurentcric     caiuil, 

nolochord,  and  niesohlast. 

notochord,  and  incsol)last. 

(Last  two  cuninic  '^ing.) 

(Last  two  ct:mnK'ncing.) 

3> 

Outjjruwth  of  tail. 

Commencing  elongation  uf 
body  of  embryo,  and  escape 
from  vitelline  membrane. 

4. 

Continued  growth  of  tail. 

Continued  elongation  of  em- 
brvo. 

5> 

Formation  of  stomudceum  and 

Formation  of  mouth,  and  com- 

atrial involutions. 

mencing  perforation  of  gill- 
clefts. 

6. 

Escape   from   vitelline  mem- 

Continued   formation    of    gill- 

brane. 

clefts  and  outgrowth  of  tail 
{i.e.  post-anal  section  of 
trunk). 

7. 

Conmiencing    perforation    of 

Formation  of  longitudinal  atrial 

gill-clefts. 

involution. 

8. 

Metamorphosis  and  commenc- 
ing   elongation    of    body 

Metamorphosis. 

proper. 

Of  course  the  above  table  has  no  concern  with  the 
actual  time  (hours  and  clays)  from  the  commencement 
of  the  development  at  which  such  and  such  an  event 
occurs.  The  type  of  Ascidian  referred  to  in  the  above 
description  is  a  simple  Ascidian  like  Ciona  or  Phallusia. 

The  above  table  also  shows  how  the  development  of 
the  Ascidian  and  of  Amphioxus  moves  along  parallel 
lines  up  to  a  certain  point,  and  then  at  the  time  of  the 
outgrowth  of  the  tail  in  the  embryo  of  the  former  and  the 
hatching  of  the  embryo  of  the  latter,  divergences  set  in. 


hi 


214 


THE  ASCIDIAXS. 


It  has  long  been  recognised  that  the  development  of  an 
Ascidian  is  much  abbreviated  in  comparison  with  that  of 
Amphioxus,  since  in  the  former  it  neither  comes  to  the 
formation  of  a  ciliated  embryo  nor  to  the  production  of 
archentcric  pouches  for  the  mesoderm.  One  of  the  chief 
evidences,  however,  of  abbreviation  in  the  Ascidian  devel- 
opment is  the  precocious  formation  of  the  larval  tail. 


I J 


m 


\  t 


Formation  of  Alinicntary  Canal  and  Hatching  of  Larva. 

When  the  enteric  cavity  of  the  Ascidian  embryo  begins 
to  grow  in  length  so  as  to  give  rise  to  the  stomach  and 
intestine,  which  it  does  shortly  after  the  appearance  of 
the  atrial  involutions,  there  is  only  one  resource  open  to 
it  on  account  of  the  limited  space  in  which  it  lies,  and  that 
is  to  double  round  upon  itself.  This  it  accordingly  does. 
As  the  growth  progresses,  the  posterior  dorsal  angle  of 
the  enteric  cavity  bends  sharply  downwards  on  the  right 
side,  and  then  upwards  and  slightly  forwards  on  the  left 
side,  ending  at  first  blindly  in  the  vicinity  of  the  left  atrial 
sac.  In  this  way  the  four  divisions  of  the  alimentary 
canal  become  established  ;  namely,  pharynx  or  branchial 
sac,  oesophagus,  stomach,  and  intestine.     (Cf.  Fig.  105.) 

By  the  time  these  changes  have  taken  place,  the  embry- 
onic development  is  at  an  end,  and  the  larva  is  ready  to 
hatch.  By  spasmodic  jcrkings  of  its  tail,  the  larva  finally 
succeeds  in  bursting  the  egg-follicle  or  vitelline  membrane 
in  which  it  has  been  hitherto  enclosed,  and  so  escapes 
into  the  open  sea. 

Clavclina  and  Ciona. 

While  the  development  of  most  forms  of  Tunicata  is  re- 
ducible to  a  common  type,  yet  the  details  vary  within  very 
wide  limits   in  different    genera.     The  tendency  here,  as 


V.  •■^i     H 


\m  f- 


AXATOA/y  AND   DEVELOPMEXT. 


215 


elsewhere,  is  to  abbreviate  the  development  by  omitting 
certain  ontogenetic  processes,  and  so  arriving  at  the  de- 
sired end,  as  it  were,  by  a  short  cut. 

One  of  the  most  impressive  instances  of  such  an  abbre- 
viated development,  and  one  which  can  be  demonstrated 
with  the  utmost  certainty,  is  afforded  by  the  genus  Clavc- 
lina,  in  contrasting  it  with  the  closely  allied  genus  Ciona. 

Clavelina  (see  Fig.  96)  is  an  Ascidian,  provided  at  its 
base  with  creeping  processes  or  stolons  containing  a  lumen 
continuous  with  the  body-cavity,  by  which  it  adheres  to 
rocks  and  weeds.  Buds  are  formed  from  the  stolon,  which 
grow  up  into  new  individuals  precisely  like  the  parent  form 
which  developed  from  the  Q%^,  and  so  a  colony  is  produced. 

Ciona  also  has  similar  basal  processes  of  the  test,  con- 
taining prolongations  of  the  original  body-cavity,  but  no 
buds  are  produced. 

In  Clavelina,  the  embryonic  development,  up  to  the  time 
of  the  hatching  of  the  larva,  takes  place  inside  the  peri- 
branchial  chamber  of  the  parent,  which  becomes  converted 
into  a  kind  of  brood-pouch. 

In  Ciona,  the  eggs  are  extruded  into  the  water,  where 
they  are  fertilised  by  the  simultaneous  extrusion  of  sper- 
matozoa from  the  same  individual.  Finally,  in  Clavelina 
the  egg  is  much  larger  and  contains  more  food-yolk  than 
that  of  Ciona. 

We  see,  therefore,  that  in  these  two  genera  the  egg  is  at 
the  outset  subjected  to  different  sets  of  conditions,  both 
internally  and  externally. 

METAMORPHOSIS   OF   CIOXA   INTESTINALIS. 

Three  stages  in  the  metamorphosis  of  the  larva  of  Ciona 
intcstinalis  are  shown  in  Fig.  105.  First,  there  is  the  free- 
swimming  larva,  which,  after  a  pelagic  existence  of  one  or 


/-iS 


2l6 


THE  ASCIDIANS. 


wt 


m 


bftj. 


'*; 


perhaps  two  days'  duration,  is  on  the  point  of  fixing  itself 
to  a  foreign  object  by  means  of  the  sticky  secretion  of  its 
three  adhering  papillae. 

This  larva  possesses  features  which  v/e  have  not  yet 
considered.  Let  us  give  our  attention  in  the  first  place 
to  the  tail, 

Vacuolisation  of  the  Notochord. 

The  vacuolisation  of  the  notochordal  tissue,  which  was 
described  above  for  Arrphioxus,  has  already  proceeded  to 
such  an  extent  that  there  is  no  longer  any  trace  of  cellu- 
lar structure  in  the  centre  of  the  notochord.  It  is  entirely 
filled  with  a  perfectly  colourless  substance,  probably  of 
gelatinous  consistency,  while  the  nuclei  have  been  dis- 
placed entirely  from  the  centre  and  can  be  seen  to  lie 
closely  pressed  against  the  dorsal  and  ventral  sides  of  the 
sheathing  membrane  of  the  notochord  (Fig.  105  A). 

There  is  one  respect  in  which  the  above  vacuolisation 
of  the  cells  of  the  notochord  differs  considerably  from 
the  corresponding  process  in  Amphioxus  and  the  higher 
Vertebrates. 

Whereas  in  the  latter  forms  the  vacuoles  appear  inside 
the  individual  cells,  —  in  other  words,  are  intracellular,  —  in 
the  Ascidian  tadpole  they  occur  between  the  cells,  and  are 
therefore  intercellular.  This  was  first  made  out  by 
Kowalevsky,  and  can  readily  be  observed.  (Cf.  Fig.  102.) 
The  intercellular  spaces  separate  the  cells  which  were 
previously  fitted  accurately  together,  end  to  end,  and, 
gradually  increasing  in  size,  they  eventually  flow  together 
and  so  constitute  a  continuous  space,  while  the  cells  with 
their  nuclei  become  thrust  aside. 

Assuming  that  the  vacuoles  contain  a  more  or  less  fluid 
substance   s'  .reted   by  the  protoplasm  of  the  cells,  the 


AX.  I  TOM  y  AND  DEVELOPMEXT. 


217 


above  difference  in  the  vacuolisation  of  the  notochordal 
tissue  in  Amphioxus  and  the  Ascidian  larva  would  resolve 
itself  into  saying  that  the  secretion  was  retained  inside 
the  cells  in  the  one  case,  and  deposited  outside  them  in 
the  other. 

Mesenchyme  and  Body-cavity. 

The  endoderm  cells  of  the  tail,  which  formed  at  first  a 
solid  cord  below  the  notochord,  have  now  become  con- 
verted into  loose  corpuscles,  which  have  mostly  floated 
out  of  the  tail  into  the  hinder  portion  of  the  body-cavity, 
and  have  become  indistinguishable  from  the  mesoderm- 
cells.  The  latter  are  beginning  to  lose  their  compact  dis- 
position in  the  form  of  the  two  mesodermic  bands,  espe- 
cially in  the  hinder  region,  and  to  be  scattered  about  in  the 
body-cavity. 

The  body-cavity  of  the  young  Ascidian  is  not  unre- 
servedly homologous  with  that  of  Amphioxus,  on  account 
of  this  remarkable  behaviour  of  the  mesoderm.  The 
cavity  does  not  arise  in  the  midst  of  the  mesoderm  by  a 
splitting  apart  of  its  component  cells,  but  it  is  simply 
produced  by  a  separation  of  the  endoderm  from  the  ecto- 
derm, the  two  layers  being  at  first  in  contact  at  the  sides 
and  below ;  in  fact,  everywhere,  except  where  the  dorsal 
nerve-tube  intervenes. 

In  the  cavity  thus  produced  between  ectoderm  and 
endoderm  the  mesodermic  bands  at  first  lie  freely,  and 
then  their  component  cells  break  away  from  their  compact 
association  and  float  about  the  cavity  in  the  form  of 
scattered  corpuscles,  known  collectively  as  mescncliyine. 

This  mesenchyme  later  gives  origin  to  the  muscula- 
ture of  the  body  proper  of  the  Ascidian,  and  als ;  to 
the  definitive  blood-corpuscles,  genital  organs,  and  renal 


//iSi 


2l8 


THE  ASCIDIAXS. 


"i. . 


vesicles.*  All  these  structures  are  differentiated  from  the 
loose  mesenchyme  cells,  all  of  which  at  first  course  round 
about  the  body  of  the  young  Ascidian  like  blood,  being 
kept  in  motion  by  the  beating  of  the  heart. 

In  the  stage  shown  in  Fig.  105  A  the  mesodermic  bands 
are  still  fairly  compact  in  front,  having  extended  them- 
selves anteriorly  at  the  sides  of  the  enteron  by  interstitial 
growth. 

Prceoral  Body-cavity  and  Piceoml  Lobe. 

When  the  larva  first  hat:hes,  the  endoderm  and  ecto- 
derm are  in  contact  with  one  another  at  the  anterior 
extremity  of  the  body,  just  as  they  are  in  the  earlier 
stages.  (Cf.  Fig.  102.)  Soon,  however,  the  ectoderm, 
with  the  adhering  papillce,  springs  away  from  the  endo- 
deriii  at  this  point,  leaving  a  space  into  which  the  two 
lateral  mesodermic  bands  force  their  way. 

In  this  way  a  special  anterior  portion  of  the  body-cavity, 
prffioral  and  pra^enteric,  is  produced,  and  is  at  first  com- 
pletely filled  by  a  compact  mass  of  rounded  cells  derived 
from  the  mesodermic  bands. 

The  end  of  the  body  of  the  larva  at  which  the  adhering 
papillcE  are  placed  of  course  corresponds  to  the  tip  of  the 
snout  in  Amphioxus. 

Just  as  Amphioxus  burrows  into  the  sand  with  its  snout, 
so  the  Ascidian  larva  fixes  itself  to  the  surface  of  a  rock 
or  weed  by  its  snout.  The  anterior  or  pra^oral  portion  of 
the  body-cavity,  of  which  we  have  just  traced  the  origin, 
is,  and  subsequently  becomes  in  a  still  more  pronounced 
way,  the  cavity  cf  t/ic  snout,  ox  prceoral  lobe. 

*  The  pericardium  arises  ventrally  from  the  endodermic  wall  of  the  liran- 
chial  sac,  and  the  heart  is  formed  l)y  an  infolding  of  the  dorsal  wall  of  the 
pericardium. 


u 


AXATOMY  AXD  DEVELOPMEXT. 


219 


Fig.  105. —  Metamorphosis  of  Cioiia  iiitestiihiln ;  above  is  repifsentcd  the 
anterior  portion  of  the  free-swimming  larva  from  tiie  left  side;  on  the  left,  the 
larva,  shortly  after  fixation,  from  the  right  siilt' ;  and  on  the  right,  the  stagi:  at 
which  the  change  of  axis  commences,  from  the  left  side.     (.After  W'll.I.KV.) 

</.  Atrial  aperture.  /'.  Uranchial  sac.  cli.  Notochord.  c.  luidostyle.  /.  Organ 
of  fixation,  g.  Ganglion.  //.  .\euro|)t)re  (having  reopened  into  branchial  sac). 
/.  Intestine.  /.  Pyloric  gland.  m.  Mouth,  n.  .N'erve-tulie.  oe.  Cl'",sophagus. 
.ot>.  I'^ye.    ('/.  Otocyst.    /.  Pericardium,    s.  Stomach,    it.  Stigmata.    /.  fail. 


ummw''^  If* 


220 


Tl/Ji  ASCIDIANS. 


if =iS    i 


r; 


j'j 


Body-cavity  of  an  Ascidian  and  Coelom  of  Aviphioxus. 

We  must  now  endeavour  to  show  how  the  body-cavity 
of  the  Ascidian  can  be  brought  into  genetic  relationship 
with  the  coulom  of  Amphioxus.  The  question  of  the 
absence  of  metamerism  in  connexion  with  the  origin  of 
mesoblast  in  the  Ascidians  need  not  detain  us,  since  it  is 
so  obviously  correlated  with  their  mode  of  life.  It  may 
safely  be  asserted  that  the  Ascidian  mesoderm,  as  a  whole, 
is  homologous  with  that  of  Amphioxus  as  a  whole,  but  in 
the  details  of  its  origin  and  fate  it  is  widely  different. 

If  we  figure  to  ourselves  the  coelomic  epithelium  of 
Amphioxus  losing  its  character  as  a  membrane  and  break- 
ing up  into  its  constituent  cells,  which  would  then  lie  loosely 
in  the  body-cavity,  we  should  have  essentially  the  same 
condition  of  things  as  in  the  Ascidians.  There  are  numer- 
ous precedents  in  the  animal  kingdom  for  such  a  disinte- 
gration of  an  epithelial  membrane. 

A  most  perfect  instance  of  it  has  been  described  by  Dr. 
R.  VON  Erlanger  *  in  connexion  with  the  origin  of  the 
mesoderm  in  the  fresh-water  snail,  Paliidina  vivipara.  Here 
the  mesoderm  appears  at  first  in  the  form  of  a  median 
bilobed  archenteric  pouch  of  relatively  large  din^ensions. 
Soon,  however,  the  cells  forming  the  wall  of  the  pouch  begin 
to  assume  irregular  shapes,  and  so  disturb  the  contour  of 
the  epithelium,  and  eventually  they  break  apart  entirely 
and  fill  every  nook  and  corner  of  the  available  space  with 
a  loose  mcscncJiynic.  Similar  out- wanderings  of  cells  from 
an  epithelial  wall,  though  not  often  of  such  a  complete 
character  as  the  instance  above  cited,  are  by  no  means 
infrequent. 

*  Z,iir  Enhvicklung  der  Paludina  vivipara.     Parts  I.  and  II.     Morpholo- 
gisches  Jahrbuch,  XVII.     1S91. 


AXATOMY  AXD   DEVEl.OFMEXT. 


221 


A  striking  example  is  afforded  by  the  body-cavity 
of  the  worm-like  Ba/anoglossitSy  of  which  we  shall  speak 
later. 

Here,  according  to  Bateson,  the  cells  lining  the  cavity 
are  continually  budding  off  daughter-cells,  which  fall  into 
the  cavity,  and  eventually  almost  entirely  fill  it  up  with 
mescncJiymatoiis  tissue.  In  this  case,  therefore,  mesen- 
chyme and  an  epithelial  wall  coexist. 

Similarly,  the  epithelial  sclerotome  of  Amphioxus  is  rep- 
resented by  a  viesenehymatous  sclerotome  in  the  higher 
Vertebrates.  It  is  not  necessary  to  multiply  instances, 
but  many  others  could  be  adduced. 

If,  now,  this  disintegration  of  parietal  and  visceral  layers 
of  the  mesoderm,  which  we  have  imagined  above  to  take 
place  in  the  ontogeny  of  an  animal  like  Amphioxus,  be 
supposed  to  be  thrown  back  in  the  development,  or,  in 
other  words,  abbreviated  to  such  an  extent  that  the  pre- 
liminary formation  of  a  continuous  coelomic  epithelium  no 
longer  takes  place,  we  should  have  precisely  those  condi- 
tions which  we  actually  find  in  existing  Ascidians. 

As  in  the  cases  above  quoted  for  purposes  of  illustra- 
tion, so  in  the  Ascidians  the  niesenchymatous  condition 
undoubtedly  originated  ancestrally  from  what  we  may  call 
an  epithelial  condition. 

In  the  Ascidians  we  may  conclude,  therefore,  that  while 
ontogenetically  the  mesenchymatous  condition  is  to  all 
intents  and  purposes  primary,  from  a  phylogenetic  point 
of  view  it  is  pre-eminently  secondary  or  cenogenetic. 

Having  made  the  reservations  implied  in  the  above 
statements,  we  may  confidently  assert  that  as  a  whole 
the  body-cavity  of  the  Ascidians  is  homologous  with  the 
coelom  of  Amphioxus,  and  we  may  define  the  former  as 
a  coelom  in  which  the  cells,  instead  of  associating  together 


//i^/» 


222 


THE  ASCIDIAXS. 


m 


to  form  a  lining  membrane  round  the  cavity,  remain 
independent  of  o  le  another  and  scattered  about  inside  the 
cavity. 

Fixation  of  the  Ascidian  Larva. 

When  the  larva  first  fixes  itself  to  some  available  sui  ""ace, 
the  tail  remains  for  a  time  stretched  straight  out  and 
almost  motionless,  giving  perhaps  an  occasional  twitch. 
Soon  the  tail  is  observed  to  become  shorter  and  to  finally 
disappear,  having  been  drawn  within  the  body  proper  of 
the  young  Ascidian.  The  entire  tail,  with  the  whole  of 
the  notochord,  musculature,  and  caudal  portion  of  nerve- 
tube,  becomes  thus  retracted  and  invaginated  into  the 
posterior  region  of  the  body-cavity,  where  it  forms  a  coiled 
amorphous  mass,  which  goes  through  a  gradual  series  of 
histolytic  changes,  and  is  finally  absorbed  by  being  dissolved 
in  the  fluid  of  the  body-cavity  (Fig.  105  B). 

By  the  time  the  tail  has  been  completely  drawn  up  into 
the  body,  the  organ  of  fixation  or  snout,  as  we  have  called 
it  above,  becomes  drawn  out  into  a  long  probosciform 
structure  in  a  line  with  the  long  axis  of  the  body.  Its 
cavity  is  no  longer  completely  filled  with  mesoderm-cells 
as  it  was  at  first  (Fig.  105  A),  but  it  has  become  so  volu- 
minous that  its  contained  cells  are  loosely  scattered  about 
(Fig.  105  B\  In  the  concluding  chapter  we  shall  endeav- 
our to  show,  what  has  been  already  implied,  namely, 
that  the  organ  of  fixation  is  seen  to  the  best  possible 
advantage  from  a  morphological  point  of  view  in  the 
species  now  under  consideration,  viz.  Ciona  intcstinalis, 
and  that  it  is  homologous  with  the  praeoral  lobe  (snout)  of 
Amphioxus,  including  under  that  term  both  the  praeoral 
body-cavity  and  the  praeoral  pit,  and  further  that  it  is 
homologous  with  the  proboscis  of  Balanoglossus. 


t  i'- 


Il"« 


.LV.i70.Vy  .IXD   DEVELOPMEXT. 


223 


At  the  stage  shown  in  Fig.  105  A,  the  lumen  of  the 
alimentary  canal  is  extremely  reduced,  and  in  many  places, 
as  in  the  region  of  the  endostyle,  c,  its  opposite  walls  are 
in  actual  apposition,  so  that  the  lumen  at  these  points  is 
almost  obliterated. 

This  temporary  reduction  of  the  lumen  of  the  alimentary 
canal  is  due  to  the  narrow  space  into  which  it  has  to  be 
compressed,  combined  above  all  with  the  relatively  enor- 
mous size  of  the  cerebral  vesicle,  which  exercises  a  great 
pressure  on  the  subjacent  dorsal  wall  of  the  branchial  sac. 
It  may  be  added  that  the  larva  of  Ciona  does  not  take  in 
food  independently  until  after  fixation. 


Reopening  of  Nciivopore ;  Degeneration  of  Cerebral  Vesicle; 
Formation  of  Definitive  Ganglion. 

One  of  the  most  obvious  features  of  the  metamorphosis 
is  the  rapid  expansion  undergone  by  the  enteric  and  body 
cavities  and  the  no  less  rapid  degeneration  of  the  cerebral 
vesicle.  This  expansion,  by  relieving  the  crowded  char- 
acter of  the  various  parts,  facilitates  greatly  the  study  of 
the  changes  which  take  place  in  the  internal  organisation. 

The  neuropore,  which  we  have  described  above  as  having 
closed  up  at  an  early  period,  now  reopens  again  and  places 
the  neural  tube  —  that  is  to  say,  as  much  of  it  as  remains 
after  the  atrophy  of  the  tail  —  in  open  communication 
with  the  base  of  the  buccal  tube  (Fig.  105  B,  ;/). 

The  spacious  cavity  of  the  cerebral  vesicle  has  vanished, 
and  its  walls  have  undergone  disintegration,  and,  except 
for  a  portion  of  the  dorsal  wall  which  becomes  converted 
into  another  channel,  are  now  represented  by  a  mass  of 
histolytic  residua  filling  the  original  cavity  of  the  vesicle 
and  lying  below  the  anterior  portion  of  the  nerve-tube. 


/-iSii 


224 


THE  ASCiniAXS. 


m  i 


This  remnant  of  the  cerebral  vesicle  of  the  larva  with  its 
sense-organs  becomes  eventually  absorbed,  and  the  eye  and 
otolith  may  often  be  found  floating  about  the  body-cavity 
with  the  ordinary  mesenchyme-cells,  and  occasionally  they 
can  be  seen  actually  passing  through  the  heart. 

The  anterior  portion  of  the  nerve-tube  itself,  which  now 
opens  into  the  base  of  the  buccal  tube  or  stomodoeum,*  is 
derived  from  a  portion  of  the  dorsal  wall  of  the  original 
cerebral  vesicle  which  was  constricted  off  from  the  latter  in 
the  form  of  a  narrow  tube  slightly  to  the  left  of  the  mid- 
dorsal  line  (Fig.  105  /?,  n). 

Subsequently  the  cells  forming  the  dorsal  wall  of  this 
portion  of  the  nerve-tube  proliferate  and  form  a  solid 
thickening  which  becomes  the  definitive  ganglion  of  the 
adult  (Figs.  105  C,  106,  and  107,  g). 

The  lumen  of  the  nerve-tube  behind  the  region  of 
the  definitive  ganglion  finally  becomes  obliterated  by  the 
mutual  approximation  of  its  constituent  cells,  and  that 
portion  of  the  primitive  nerve-tube  which  in  the  larva  lay 
between  the  cerebral  vesicle  and  the  root  of  the  tail  is  thus 
represented  in  the  adult  by  a  solid  "  cordon  ganglionnairc 
viscih'aV  (van  Beneden  and  Julin)  which  starts  from  the 
posterior  end  of  the  adult  cerebral  ganglion,  and,  proceed- 
ing along  the  dorsal  side  of  the  pharynx  above  the  dorsal 
lamini,  becomes  lost  among  the  viscera.  (Cf.  Figs.  96, 
105,  and  107.) 

Below  and  in  front  of  the  definitive  ganglion,  which 
finally  becomes  quite  separate  from  the  dorsal  wall  of  the 
neural  tube,  the  lumen  of  the  latter  persists  and  becomes 

*  According  to  renewed  observations  on  Ciona,  I  find  that  the  neuropore 
reopens  into  the  buccal  tube  precisely  in  the  line  of  junction  of  the  stomo- 
da-um  with  the  wall  of  the  branchial  sac,  so  that  its  upper  margin  is  continu- 
ous with  the  (ectodermic)  stomodoeal  epithelium,  and  its  lower  margin  with 
the  (endodermic)  branchial  epithelium.     (See  below,  V.) 


ANA  TOM  y  :L\D   DEVELOPMEXT. 


2^S 


C.iA^ 


by  subsequent  extension  the  lumen  of  the  subneural  gland 
and  its  duct. 

Thus  the  anterior  portion  of  the  primitive  neural  tube, 
having  become  constricted  off  from  the  cerebral  vesicle 
of  the  larva,  and  having  given  rise  by  proliferation  from 
its  dorsal  wall  to  the  definitive  ganglion,  becomes  bodily 
converted  into  that  structure  which  we  shall  call,  in  agree- 
ment with  JuLiN,  the  hypopJiysis. 

The  opening  of  the  latter  into  the  base  of  the  buccal 
tube  becomes  the  dorsal  tubercle  of  the  adult.     Finally,  at 
a  much  later  stage,  the  glandular  portion  of  the  hypophy- 
sis arises  by  proliferation  of 
spongy  tissue  from  the  ven- 
tral wall  of  that  portion  of 
the    neuro-liypophysial   tube 
which  lies  immediately  be- 
low the  ganglion. 

A  section  through  the 
cerebral  vesicle  of  a  larva 
of  Distaplia,  a  colony-build- 
ing Ascidian,  showing  the 
hypophysis    in    process    of       „.        ,      ^       ,       •      ,.       .. 

■'  ^     ^    ■'  '  Fig.  106.  —  Frontal  section  through 

being     constricted    off     from    cerebral  vesicle  of  a  larva   of  Ihstaplta 

magnihirva,  to  show  the  origin  of  the 
ganglion  and  hypophysis.  (After  HjORT ; 
combination  of  two  figures.) 

In  the  larva  of  Distaplia,  the  hypophy- 
dition  of   things  generally  is    sis    opens    into    the    branchial    sac    be- 

,.-.  ^       f.  ,     ^    hind  the  stomodccuin. 

very     different      from      what  ,,,_  cerebral   vesicle,     ec.  Ectoderm. 

obtains    in    Ciona,    but    it    is    ''"•  Kndoderm.     ,^^  Ganglion,      hy.  Hy- 
pophysis (neuro-hypophysial  tube). 

introduced     to     show     the 

essential  similarity  in  the  mode  of  origin  of  the  hypophy- 
sis in  this  form,  as  observed  by  Dr.  Johan  Hjort. 

In  Distaplia,  as  is  also  the  case  to  a  less  extent  in 
Clavelinay  the  ganglion  begins  to  develop  from  the  wall 


the  vesicle,  is  given  in  Fig. 
106.    In  this  jrenus  the  con- 


j^. 


mmm 


226 


THE   ASCIDLWS. 


f-iJ 


of  the  neuro-hypophysial  tube  while  the  latter  is  still  in 
connexion  with,  and  therefore  before  the  atrophy  of,  the 
cerebral  vesicle,  thus  indicating  a  hastening  in  the  devel- 
opment as  compared  with  Ciona. 

The  convexity  caused  in  the  dorsal  wall  of  the  branchial 
sac  by  the  pressure  of  the  cerebral  vesicle  persists  as  the 
anterior  portion  of  the  dorsal  lamina,  and  in  many  or  most 
simjjlc  Ascidians  becomes  grooved,  forming  the  cpibmti- 
cliial groove  of  Julin  (Fig.  97).  At  present  it  is  merely  a 
ridge,  the  cpibmucJiial  ridge. 

In  Fig.  105  (7  the  proximal  (oral)  end  of  the  endostyle, 
<•,  is  seen  to  be  connected  with  the  epibranchial  ridge  by 
the  peripharyngeal  baud,  which  we  have  already  described 
in  the  adult.  It  apparently  arises  ///  situ  by  simple  spe- 
cialisation of  the  cells  forming  the  epithelial  wall  of  the 
pharynx  at  this  point. 

Pritnary   Topographical  Relations  and  Change  of  Axis. 

It  must  be  especially  noted  that  the  long  axis  of  the 
young  Ciona  for  some  time  after  fixation  is  identical  with 
that  of  the  tailed  larva,  and  therefore  the  primary  topo- 
graphical relations  of  the  various  parts  are  maintained  at 
the  stage  shown  in  Fig.  105  B,  and  we  can  accordingly  make 
use  of  this  stage  in  which  different  structures  are  much 
clearer  than  in  the  free-swimming  larva  for  the  purpose  of 
describing  the  primary  topography,  which  is  of  the  utmost 
importance  when  it  is  desired  to  institute  a  comparison 
with  Amphioxus. 

Since,  as  we  have  seen,  the  details  of  the  embryogenetic 
processes  differ  in  many  respects  widely  from  what  occurs 
in  Amphioxus,  we  are  inevitably  compelled  to  rely  to  a 
very  large  extent  on  topographical  relations  in  order  to 
estimate  the  homology  of  this  or  that  structure  in  the 


A.y.i/o.vy  A.vj)  Di-.VELovMi-.xr. 


227 


Ascidians  and  in  Amphioxus.  Fortunately  there  is  one 
structure  as  to  whose  comi)lete  homolo<;y,  in  the  Urochorda 
(Tunicata),  on  the  one  hand,  and  the  Cephalochorda,  on  the 
other,  no  one  entertains  a  doubt,  and  that  is  the  cndostylc. 
We  thus  have  in  the  endostyle  a  firm  basis  upon  which  to 
ground  our  deductions. 

In  the  larva  and  in  the  young  Ascidian  before  the 
primary  long  axis  has  been  disturbed  in  the  way  which  we 
shall  shortly  describe,  the  endostyle  is  the  most  anterior 
endodermic  structure  in  the  body,  and  lies  dorso-ventrally 
at  right  angles  to  the  long  axis  of  the  body  (Fig.  105  A 
and  B,  c). 

As  described  above  in  the  larvae  of  Amphioxus,  particu- 
larly in  the  younger  larv?c(see  Figs.  64  and  73),  the  endo- 
style, though  lying  asymmetrically  on  the  right  side,  being 
involved  in  the  general  asymmetry  of  the  larva,  is  quite 
anterior  in  position,  in  front  of  all  the  gill-slits  and  partly 
in  front  of,  though  also  partly  opposite,  the  mouth  (on 
account  of  its  asymmetry),  and  almost  at  right  angles  (see 
especially  Fig.  64)  to  the  long  axis  of  the  body.  As  there 
is  only  a  short  stretch  of  simple  endoderm  in  front  of  the 
endostyle  in  the  larva  of  Amphioxus,  we  may  describe  it 
as  the  most  anterior  differentiated  endodermic  structure 
in  the  larva,  thus  corresponding  with  remarkable  precision 
to  the  condition  described  above  in  the  larval  and  newly 
fixed  Ascidian. 

In  the  middle  of  the  wall  of  the  branchial  sac  in  Fig. 
105  B  are  seen,  somewhat  in  front  of  and  below  the  atrial 
aperture,  «,  of  this  side,  two  lens-shaped  structures  whose 
slightly  concave  sides  face  each  other.  These  are  the 
borders  of  the  two  first-formed  primary  branchial  stigmata 
or  gill-clefts.  Their  actual  openings  into  the  atrial  chamber 
are  at  present  so  small  that  they  can  hardly  be  seen  in 


/J^ 


p-'- 


.1 

Vi 


'ir;.:... 


228 


77/A   ASCIDIAXS. 


surface-view,  but   they  are  situated  at   the  inner  or  con- 
cave sides  of  the  two  thickenings.     On  either  side  of  the 

latter  can  be  seen  the 
ordinary  cavity  of  the 
pharynx  proceeding  to- 
wards the  oesophagus. 
At  a  later  stage  the 
openings  of  the  two 
first-formed  stigmata 
become  distinctly  visi- 
ble (Fig.  iOSC).  Mean- 
while a  change  of  axis 
is  taking  place  in  the 
body  of  the  young 
Ascidian. 

During  the  extraor- 
dinary change  of  axis 
which  we  are  about 
to  describe  the  probos- 
ciform  praeoral  lobe 
(snout,  organ  of  fixa- 
tion) remains  station- 
ary, and  the  rest  of  the 
body  actually  rotates 
through  an  angle  of  go 


i  !i' 


illlj 


Fig.  107.  —  Young  Cio}ia  inteatinalis  after 
the  completion  of  the  change  of  axis ;  from  the 
left  side.     (After  Wl  I.I.KY.) 

/,  IV\  Primary  stigmata,      a.  Anus,  situated      .  .  , 

immediately  below  the  left  atrial  aperture,  end.  dCgrCCS,  USmg  the  Or- 
Enddstyle.  f.  Organ  of  fixation.  ^1^.  Ganglion,  cran  of  fixition  aS  1. 
hy.  Hypophysis.    /;//.  Intestine.    I. at.  Left  atrial    '^ 

a])erture.  l.m.  Longitudinal  muscle,  in.  Mouth,  pivot  about  which  it 
oes,    U'Lsophagus.      p.b.    Perijiharyngeal     band.  T       TT"  r 

py.  Pyloric  gland,  st.  Stomach.  /.  Coronary  t^rUS.  In  rig.  IO5  (. 
tentacles,  v.n.  Visceral  nerve  (cordon  ganglion-  \\^P  rotation  which 
naire  visceral). 

takes  place  very  gradu- 
ally is  only  half  performed ;  while  in  Fig.  107  it  is  complete. 
The  method  of  growth  by  which  this  rotation  takes  place 


hu 


I^^^   ■    '  * 


AXATOMY  .LXD   DEVELOPMEXT. 


229 


is  of  a  very  singular  character,  and  it  is  difficult  to  define 
it  in  precise  terms. 

In  this  way  then  the  endostyle  (and  branchial  sac 
generally)  comes  to  be  placed  at  right  angles  to  its  primary 
position. 

Since  in  Amphioxus  the  endostyle  altered  its  primary 
axis  by  a  process  of  independent  growth  while  the  long 
axis  of  the  pharynx  was  constant  throughout  the  develop- 
ment, we  find  that  here  again,  as  in  so  many  previous 
instances,  the  details  by  which  similar  end-results  are 
arrived  at  are  widely  dissimilar. 

This  complete  change  of  axis  by  which  the  pra^oral  lobe 
(organ  of  fixation)  becomes  placed  at  the  posterior  extrem- 
ity of  the  body  can  only  be  regarded  as  a  cenogenetic 
feature.* 

It  is  therefore  chiefly  to  the  primary  relations  which  the 
various  structures  bear  to  one  another,  before  the  change 
of  axis,  that  we  must  turn  for  purposes  of  comparison.  If 
we  do  this,  we  find  that  the  following  sequence  of  organs 
obtains  as  well  in  the  larva  of  Amphioxus  as  in  the  newly 
fixed  larva  of  Ciona ;  namely:  i,  praeoral  lobe;  2,  endo- 
style ;  3,  mouth  ;  4,  gill-clefts. 

Formation  of  Additional  Branchial  Stigmata, 

After  the  change  of  axis  of  the  body,  the  long  axes  ot 
the  stigmata  lie  transversely.  In  their  further  growth 
they  go  on  elongating  in  the  same  (transverse)  direction, 
and  after  they  have  attained  a  certain  size  their  ventral 
ends  —  that  is  to  say,  the  ends  nearest  the  endostyle  —  bend 
round  towards  each  other,  and  from  each  of  the  two  first- 

*  It  ^^oes  without  saying  that  the  primary  long  axis  of  the  Ascidian  larva  is 
homologous  with  the  long  axis  of  Amphioxus. 


(■Shi 


230 


THE  ASCIDIANS. 


iff:- 


^r^' 


formed  stigmata  a  minute  portion  becomes  gradually  con- 
stricted or  nipped  off.  Thus  between  and  cut  off  from  the 
two  original  stigmata,  there  come  to  lie  two  intermediate 
stigmata  of  much  smaller  size.     (Cf.  Fig.  107.) 

In  this  way,  then,  in  Ciona,  we  arrive  at  the  stage  with 
four  branchial  stigmata  on  each  side  of  the  pharynx.  For 
convenience  we  shall  refer  to  these  by  the  Roman  nu- 
merals, I.,  II.,  III.,  and  IV.  It  is  a  remarkable  fact  that 
II.  and  III,  do  not  arise  by  new  perforations,  but  are  cut 
off  from  I.  and  IV.  respectively. 

On  account  of  the  close  relations  which  the  two  first- 
formed  stigmata,  I.  and  IV.,  bear  to  one  another  during 
the  production  of  the  intermediate  stigmata,  their  ventral 
extremities  coming  into  contact  and  apparently  some- 
times fusing  together  so  that  II.  and  III.  might  almost 
be  described  as  a  joint  production  of  I.  and  IV.  rather  than 
as  entirely  independent  offshoots,  one  is  forced  to  the 
conclusion  that  the  two  first-formed  stigmata  themselves, 
though  they  actually  appear  simultaneously  as  separate 
perforations,  in  reality  represent  the  two  halves  of  a 
single  primitive  gill-slit  divided  into  two  by  a  tongue- 
bar.  If,  moreover,  we  examine  the  exact  origin  of  these 
two  stigmata  (I.  and  IV.)  by  means  of  transverse  and 
horizontal  sections,  we  may  become  convinced  that  such 
is  indeed  the  case  ;  namely,  that  they  represent  the  two 
halves  of  a  primitive  gill-slit  which,  on  account  of  the 
precocious  formation  of  the  tongue-bar  between  them, 
become  perforated  separately. 

For  the  formation  of  any  two  or  more  consecutive  gill- 
slits,  we  usually  expect  to  find  separate  endodermic  pockets 
or  pouches  of  greater  or  less  depth  growing  out  towards 
the  ectoderm.     (Cf.  Figs.  72  and  92.) 

We  ought  to  find  something  analogous  to  this  in  Ciona 


AXATOMV  AXD  DEVELOl'MEXT. 


231 


if  the  two  first-formed  stigmata  had  the  value  of  indepen- 
dent gill-slits. 

Instead,  however,  of  anything  approaching  to  two  cndo- 
dermic  outgrowths,  we  find  at  the  base  of  the  atrial  invo- 
lution a  single  endodermic  ingrowth  making  its  appearance 
(Fig.  108). 

The  angles  made  by  this  ingrowth  with  the  neighbour- 
ing wall  of  the  branchial  sac  remain  in  contact  with  the 
floor  of  the  atrium,  then  fuse  with  it,  and  finally  become 


en 


at  f 

miim»mimmiirmrmJ.^m>>»»)»»»mM 


D 

'fm»»)»»mm»mmm 


at 

1 

i 


S^ 


fh 


Fig.  108.  —  Diagrams  illustrating  the  mode  of  origin  of  the  two  first-formed 
branchial  stigmata  in  Ciona.     (After  WlLI.EY.) 

at.  Atrial  involution,  ec.  Ectoderm,  en.  Endoderm.  g.s.  Stigmata,  t.b. 
Tongue-bar. 

perforated  (Fig.  108).  This  is  the  way  in  which  the  stig- 
mata, I.  and  IV.,  ari.se,  and  it  is  difficult,  if  not  impossible, 
to  interpret  the  above-mentioned  endodermic  ingrowth 
otherwise  than  as  ts.  precocious  tongac-bar. 

Even  in  Amphioxus  it  was  seen  how  the  tongue-bars 
of  the  secondary  slits  arose  relatively  much  earlier  than 
those  of  the  primary  slits.  If  they  arose  still  a  trifle 
earlier,  we  should  have  the  two  halves  of  each  slit  becom- 
ing separately  perforated,  just  as  it  hap])ens  in  Ciona. 
In  a  species  of   Balanoglossus   an   analogous    precocious 


23: 


THE  ASCIDIAXS. 


n 


-pi  j 


W 


A»h 


formation  of   tongue-bars,   before  the  perforation   of   the 
slits,  has  been  described  by  Professor  T,  H.  Morgan. 

From  what  has  been  said  above,  we  conclude  that  the 
first  four  pairs  of  primary  branchial  stigmata  of  Ciona 
(and  this  probably  applies  equally  to  many  species  of 
Phallusia)  represent  and  are  derivatives  of  one  pair  of 
primitive,  ancestral  gill-slits. 

After  a  comparatively  long  interval,  during  which  the 
intermediate  stigmata,  II.  and  III.,  increase  in  length 
transversely,  two  more  stigmata,  V.  and  VI.,  arise  at  inter- 
vals, one  after  the  other,  by  sepa- 
rate perforations  behind  those 
/  £^  ^rf**"*^  already  formed  (Fig.  109). 

On  account  of  the  independent 
origin  of  V.  and  VI.,  it  might  be 

m  ffL-fflsa**^   ^-rt#^  V'       supposed  that   they  would  have 

the  morphological  value  of  dis- 
tinct gill-slits,  and  that  we  had 
before  us  three  pairs  of  ancestral 

Fig.  109. -Primary branchial  giU'sHts  represented  by  six  pairs 
stigmata  of  the  right  side  of  a  ^f    primary   branchial     stigmata. 

young  Ciona.     (After  Willey.)  sr  j  o 

For  this  interpretation  to  hold 
good,  we  should  expect  to  find  that  in  other  forms  in  which 
six  primary  branchial  stigmata  were  produced,  their  origin 
was  either  the  same  or  reducible  to  the  same  type  as  that 
of  the  branchial  stigmata  of  Ciona. 

This,  however,  is  not  the  case,  since  I  have  found 
that  in  Molgula  manhattcnsis*  a  simple  Ascidian  which 
occurs  in  great  numbers  at  New  Bedford,  Mass.,  the  six 
primary   stigmata,    corresponding    precisely   to    those    in 


ZE 


*  My  observations  on  the  development  of  Molgula  manhattensis  were 
made  at  the  Marine  Biological  Laboratory,  at  Woods  Holl,  Mass.,  in  the 
summer  of  1893. 


i»: 


AX.i7'CU/y  .LVD  DEVELOPMEXT. 


'■IZ 


Ciona,  have  a  somewhat  different  mode  of  origin.  The 
two  first-formed  stigmata  (=  I.  and  IV.  in  Ciona)  appear 
simultaneously  as  in  Ciona.  Then  after  growing  to  a  cer- 
tain size,  they  curve  round  at  their  ventral  ends,  not  in 
opposite  directions  so  as  to  meet  each  other  as  they  do  in 
Ciona,  but  in  the  same  direction  (Fig.  no).  The  recurved 
ends  then  become  constricted  off  from  the  parent  stig- 
mata. Later  on,  a  fifth  gill-opening  arises  behind  the 
first  four  stigmata  by  independent  perforation,  and  after 


Fig.  no.  —  Diagram  illustrating  the  mode  of  origin  of  the  six  primary  bran- 
chial stigmata  of  Atolgtda  manhattensis.  The  numbers  are  placed  at  the  ventral 
ends  of  the  slits.  The  figure  is  a  combination  of  several  liitherto  unpublished 
drawings  of  different  stages  in  the  development.  /,  ///,  and  t' arose  by  separate 
perforation. 


attaining  a  certain  size,  it,  in  its  turn,  curves  round  at  its 
ventral  end,  and  eventually  the  sixth  stigmatic  opening  is 
constricted  off  from  the  fifth. 

Since  the  first  six  primary  stigmata  have  such  different 
origins  in  two  different  species,  it  is  obvious  that  in 
attempting  to  make  a  comparison  with  Amphioxus  we  can 
only  use  the  two  first-formed  stigmata,  because  they  agree 
in  the  above-mentioned   species,  and  in  many  others  in. 


/.iSf 


w 


.;!■ 


234 


7Y//S    ASC/D/AXS. 


m 


arising  simultaneously,  and  in  representing,  in  all  proba- 
bility, the  two  halves  of  a  primitive  gill-slit,  cut  in  two  by 
a  tongue-bar. 

The  stigmata  which  are  added  to  these  must,  therefore, 
be  regarded  as  secondary  modifications,  hardly  comparable 
to  the  successive  formation  of  new  gill-slits  in  Amphioxus. 

In  the  Ascidians,  therefore,  we  can  only  detect  the 
representatives  of  one  pair  of  primitive  gill-slits,  and  there 
is  every  reason  for  supposing  them  to  be  homologous  with 
the  first  pair  of  gill-slits  in  Amphioxus  as  defined  above. 

The  six  primary  stigmata  of  each  side  give  rise,  by  re- 
peated subdivision,  to  the  innumerable  stigmata  of  the 
adult,  both  in  Ciona  and  Molgula.  The  following  de- 
scription, however,  applies  more  particularly  to  Ciona. 

In  the  first  place,  the  primary  stigmata  grow  to  a  sur- 
prising transverse  length,  and  then  commence  to  divide 
into  two  equal  portions  by  small  tongue-like  projections, 
which  grow  across  the  aperture  indifferently  from  the 
anterior  or  posterior  walls  of  the  respective  stigmata,  and, 
fusing  with  the  opposite  wall,  divide  the  transversely 
elongated  slit  into  two  completely  separated  halves.  Then 
each  of  the  latter  divides  again  in  the  same  manner,  and 
so  the  process  of  subdivision  of  existing  stigmata  goes  on. 
In  this  way  six  transverse  rows  of  stigmata  arise.  These 
may  be  distinguished  as  secondary  stigmata,  since  they 
arise  by  division  from  the  primary. 

Gradually,  by  a  peculiar  process  of  growth,  the  long 
axes  of  the  secondary  stigmata  change  their  direction,  and 
instead  of  lying  transversely  they  become  directed  antero- 
posteriorly.  This  is  their  definitive  position,  and  the 
stigmata  now  go  on  rapidly  dividing  again,  and  the  num- 
ber of  transverse  rows  of  stigmata  is  in  this  way  doubled, 
trebled,  quadrupled,  etc.,  and  we  thus  arrive  at  the  adult 


jfjs-, 


ANATOMY  AND  DEVELOPMENT. 


235 


condition.  Out  of  the  multitude  of  stigmata  which  are 
present  in  the  adult  Ciona  only  four  arise  by  independent 
perforation  ;  namely,  the  primary  stigmata  I.  and  IV. 
(which  we  regard  as  the  two  halves  of  a  primitively  single 
slit)  and  V.  and  VI. 

Fh'st  Appearance  of  MtisculaUire. 

By  the  time  the  change  of  axis  of  the  entire  body  of 
the  young  Ciona  has  been  effected  the  musculature 
characteristic  of  the  adult  begins  to  put  in  an  appear- 
ance. In  Fig.  107  circular  sphincter  muscles  are  present 
round  the  buccal  and  atrial  apertures.  The  latter  are  still 
paired,  but  are  carried  by  differential  growth  dorsalwards 
at  a  later  stage,  and  finally  coalesce  together  in  the  dorsal 
middle  line  to  produce  the  single  atrial  aperture  of  the 
adult. 

One  strand  of  the  longitudinal  muscles  of  the  later 
muscular  mantle  is  likewise  to  be  seen  in  Fig.  107.  It 
tends  to  branch  dichotomously.  Posteriorly  it  is  inserted 
on  the  inner  surface  of  the  organ  of  fixation  near  the  point 
where  it  joins  on  to  the  body.  Later  new  muscle-bands 
arise  similar  to  the  first,  and  become  distributed  over  the 
body-wall  in  a  spreading  fan-like  fashion,  but  posteriorly 
they  are  all  inserted  in  the  same  region  of  the  organ  of 
fixation. 

Alimentary  Cajial  and  Pyloric  Gland. 

The  course  of  the  alimentary  canal  can  be  gathered  so 
plainly  from  the  accompanying  figures  (F'igs.  105  and  107) 
that  it  hardly  needs  a  verbal  description.  From  the 
posterior  dorsal  corner  of  the  branchial  sac  the  oesophagus 
leads  into  the  wide  stomach,  and  from  the  latter,  again, 
the  intestine,  which  often  possesses  a  strangulated  appear- 


L'Tti 


236 


TIIK  ASCW/.LVS. 


m 


m  t 


l*r^^ 


ance,    doubles    up   obliquely   forwards   to   the   left    atrial 
chamber,  into  which  it  opens  by  the  anus  (Fig.  107). 

In  the  angle  made  by  the  outgoing  intestine  with  the 
stomach,  a  blind  diverticulum  arises.  It  is  at  first  a  sim- 
ple coecum,  but  soon  begins  to  branch  (Fig.  105  C),  and 
finally  forms  an  arborescent  growth  embracing  the  in- 
testine (Fig.  107).  This  is  the  so-called  pyloric  gland, 
and  it  is  probably  homologous  with  the  hepatic  caecum  of 
Amphioxus. 

Appcndiculayia. 

It  is  generally  agreed  among  those  who  have  a  voice  in 
the  matter,  that   most  of   the  pelagic  Ascidians  (Salpa, 

Doliolum,  Pyrosoma)  are 
highly  modified  forms,  spe- 
cially adapted  to  a  pelagic 
life,  one  of  the  results  of 
which  is  that  their  repro- 
duction is  marked  by  a 
complicated  alternation  of 
generations. 

It  would,  therefore,  not 
assist  us  in  our  comparison 
with  Amphioxus  to  describe 
these  types. 

There  is,  however,  one 
family  of  pelagic  Ascidians, 
the  Appendicularice,  with  re- 
spect to  which  there  are  two 
widely  different  opinions. 

The  Appendicularize  are 
pelagic,  free-swimming  As- 

<z.Anus  .^/.  Unicellular  glands  ^..  cidiaUS,  whoSC  adult  COndi- 
Gill-slits.     //.   Dorsal    hood-like    fold    of 


Fig.  III.  —  Appendicularia  {Fritil- 
lariti)  ftircata,  from  the  ventral  surface. 
(After  Lankkster.) 


integument.     ;;/.  Mouth.    /.  Tail. 


tion  is  SO  far  similar  to  the 


.I.V.IV'OJ/y  A\D  DEVELOPMEXT. 


237 


larval  condition  of  the  fixed  Ascidians,  that  they  retain  the 
tail  as  their  organ  of  locomotion  throughout  life  (Fig.  1 1 1). 

The  tail  is  inserted  in  the  middle  of  the  ventral  surface 
of  the  body  proper,  and  is  obviously  a  mere  appendage  of 
the  latter. 

The  mouth  is  terminal  or  sub-terminal.  There  is  a  sin- 
gle pair  of  branchial  stigmata,  which  open  into  a  pair  of 
tubular  atrial  cavities,  whose  separate  external  apertures 
are  seen  in  front,  on  the  ventral  surface  behind  the  mouth. 

The  alimentary  canal  is  U-shaped,  and  the  anus  opens 
on  the  ventral  surface  to  the  right  of  the  middle  line,  some- 
times behind  and  some- 
times (according  to  the 
species)  in  front  of  the 
stigmata  (Figs,  iii, 
112).  The  endostyle 
is  always  quite  anterior 
in  position,  and  some- 
times, as  in  Fig.  112, 
removed  by  a  consider- 
able interval  from  the 

Pig.  112.  —  Diagram  of  the  organisation  of 
stigmata.  ^  species  of  Appendicularia,  from  the  right  side. 

In  the  posterior  ex-   (^'"-''-  hkrdman.) 

^  a.    Anus;    the    mdex    hne  was   accidentally 

tremity     of     the      body  drawn  about  Vs  of  an  inch  in  front  of  the  anus. 

.          ^     ,                    .  b.s.  Hranchial  sac.    ch,  Notochord.    e.  Endostyle. 

are  placed  trie   gonads,  g^  CangUon,  from  which  the  nerve-cord  proceeds 

male     and     female,     in  backwards  to  the  tail,  passmg  to  the  right  of  the 

alimentary  canal.    ,^.s.  GiU-slit.    Ii.    Heart,     int. 

close  proximity   to  one  intestine,     vt.    Mouth,     n.c.    Nerve-cord,    with 

,              ,                  .       .  ganglionic  enlargements  in  the  tail.    o/.  Otocyst; 

another,    trie    testis    in  beneath  which   the  hypophysis   opens   into   the 

front      and     the      ovary  branchial  sac     m.  Ovary.    p.b.  Peripharyngeal 

•^  band.    st.  Stomach,    te.  Testis. 

behind.     The  heart,  as 

described  by  Lankester,  is  a  unique  example  of  a  func- 
tional organ  reduced  to  the  lowest  possible  level  of  histo- 
logical structure.     It  consists  simply  of   two  cells  placed 


; 

I 

238 


THE  ASCI  1)1  AXS. 


\»k 


m 


m\-  ^f^ 


opposite  one  another  and  connected  together  by  contractile 
protoplasmic  threads,  which  keep  up  a  pulsating  motion. 

The  tail  is,  as  might  be  expected,  more  elaborately  or- 
ganised than  that  of  the  Ascidian  larva.  The  dorsal  nerve- 
cord  is  solid,  and  proceeds  backwards  from  the  ganglion, 
passing  to  the  riij^Jit  of  the  alimentary  canal  until  it  reaches 
the  tail,  along  which  it  is  continued,  lying  to  the  left  of 
the  notochord ;  it  possesses  ganglionic  enlargements  at 
intervals  in  the  tail,  from  which  nerves  pass  out. 

The  caudal  musculature  also  shows  somewhat  doubtful 
traces  of  being  segmented  in  correspondence  v^^ith  the 
ganglionic  swellings  of  the  nerve-cord. 

In  connexion  with  the  cerebral  ganglion  there  is  a 
sense-organ  in  the  form  of  an  otocyst,  with  an  enclosed 
otolith,  and  below  this  a  ciliated  pit  opens  into  the  ante- 
rior region  of  the  branchial  sac,  corresponding  to  the 
hypophysis,  or  sub-neural  organ,  of  the  fixed  Ascidians. 

According  to  one  view,  Appendicularia  is  the  living  rep- 
resentative of  the  free-swimming  ancestor  of  the  Ascidians, 

According  to  the  other  view,  it  is  less  primitive  than  the 
fixed  Ascidians,  and  was  derived  from  the  latter  by  the 
gradual  increase,  from  generation  to  generation,  of  the  du- 
ration of  the  pelagic  existence  of  the  larvae,  until  they 
ceased  to  metamorphose,  and  so  retained  the  larval  struct- 
ure throughout  life,  becoming  at  the  same  time  sexually 
mature.'"^ 

These  two  views  are,  of  course,  antagonistic,  and  the 
former  of  them  is  held  by  a  number  of  well-known  author- 
ities. As  we  are  ignorant  of  the  development  of  Appen- 
dicularia, it  is  impossible  to  decide  definitely  between  them. 

With  the  facts  which  are  at  our  disposal,  however,  the 
second  view  —  namely,  that  the  Appendicularice  represent 
Ascidian  larvng  which  have  become  secondarily  adapted  to 


■k. 


liiiiii. 


.ix.r/(K]/y  .i\j)  Di-.n-.LoPM/.xr. 


239 


a  pelagic  life,  and  have  acquired  the  faculty  of  attaining 
sexual  maturity  —  would  be  more  in  harmony  with  what 
we  know  of  the  relation  of  Amphioxus  to  the  Ascidians. 
And  it  would  seem  that  this  affinity  can  be  better  demon- 
strated through  the  comparison  of  Amphioxus,  both  adult 
and  larva,  with  a  fixed  Ascidian  like  Ciona  than  with 
Appendicularia.'* 

On  the  latter  view,  therefore,  the  so-called  metamerism 
of  the  tail  of  Appendicularia,  on  which  so  much  stress  has 
been  laid,  would  be  simply  a  secondary  elaboration  of  the 
tail  for  the  purpose  of  serving  as  a  permanent  locomotor 


organ. 


The  dorsal  nerve-cord  of  Appendicularia  was  regarded 
by  FoL  as  a  simple  peripheral  nerve.  We  have  described 
above  how  a  portion  of  the  primitive  nerve-tube  in  Ciona 
and  other  Ascidians  becomes  reduced  to  a  solid  nerve. 

It  would  be  of  the  greatest  interest  to  discover  the  mode 
of  origin  of  this  nerve-cord  in  Appendicularia. 

Abbreviated  Ontogeny  of  Clavelina. 

In  order  to  demonstrate  clearly  the  relatively  primitive 
character  of  the  development  of  Ciona  it  is  sufficient  to 
enumerate  a  few  facts  drawn  from  the  development  of 
Clavelina  as  described  by  Dr.  Oswald  Seeligek.  As 
mentioned  above,  Clavelina  is  a  near  relative  of  Ciona,  and 
m  the  adult  condition  resembles  it  very  closely  in  many 
respects. 

The  development  of  Clavelina  was  formerly  regarded  as 
being  of  a  primitive  character,  but  is  in  reality,  more 
especially  in  the  later  stages,  abbreviated  and  hastened  to 
a  remarkable  extent. 

Like  Ciona  it  possesses  in  the  adult  numerous  trans- 
verse rows  of  stigmata.     Each  opening,  however,  arises  by 


240 


THE  ASC/D/AXS. 


m-. 


an  independent  perforation,  so  that  all  those  preliminary 
onto<;enetic  processes  which  precede  the  establishment  of 
the  transverse  rows  of  stigmata  in  Ciona  are  dropped  out 
of  the  development  of  Clavelina.* 

In  Clavelina,  again,  the  change  of  axis  of  the  body 
proper  occurs  in  the  unhatched  larva ;  so  does  the  fusion 
of  the  two  atrial  apertures  to  form  the  dorsal  cloacal 
siphon.  The  longitudinal  muscles  of  the  body  proper 
commence  to  appear  m  the  free-swimming  larva,  while  the 
caudal  muscles  are  enjoying  their  highest  functional 
activity.  The  vacuolisation  of  the  notochord  does  not 
proceeii  so  far  as  in  Ciona,  since  the  cells  are  never  actu- 
ally removed  from  the  centre  of  the  notochord,  but  remain 
as  thin  discs  stretching  across  the  latter,  so  that  the 
vacuolar  spaces  do  not  become  continuous. 

The  behaviour  of  the  organ  of  fixation  in  the  larva  of 
Clavelina  is  such  that  it  could  hardly  be  recognised  as  a 
prreoral  lobe  except  in  the  light  of  Ciona. 


NOTES. 


I.  (p.  183.)  The  test  or  cellulose  mantle  of  the  Ascidians  con- 
tains great  numbers  of  cells  of  various  kinds.  These  we-e  formerly 
supposed  to  be  derived  from  the  subjacent  ectoderm  of  the  body- 
wall.  KowALKVSKV  has  recently  shown,  however,  that  the  cells  of 
the  outer  (cellulose)  mantle  of  the  Ascidians  are  derived  from 
wandering  mesenchyme-cells  which  wander  from  the  body-cavify 
through  the  ectoderm  (either  hetxueen  the  ectodermic  cells  jr 
actually  passing  through  the  individual  cells)  into  the  mantle. 

*  A  mode  of  formation  of  the  branchial  stigmata,  intermediate  between 
that  of  Clavelina  and  Ciona  or  Molgula,  has  been  described  by  (jARSTANc; 
for  Uotryllus.  In  this  genus,  the  primary  branchial  stigmata  all  arise  by  in- 
dependent perforations,  and  then  later  become  divided  up  into  the  transverse 
rows  of  stigmata.  ('  '.  uakstanc.  On  the  development  of  the  stigmata 
in  Ascidians.     Proc.  K    /.  Soc,  Vol.  LI.     1892.') 


A'07'ES. 


241 


2.  ( p.  2 1 1 . )     In  Clavelina  tlie  atrial  involutions  do  not  merely 
arise  as  minute  circular  invaginations  of  the  ectoderm,  btit  at  first 
they  appear  as  short,  though  <,uite  distinct,  longitudinal  gn)oves 
Compare  also  the  remarkal.le  longitudinal  atrial  tubes  of  Pxro.uwuu 

3.  (p.  23.S.)     There  is  another  possible  way  of  interpreting  the 
structure  and  systematic  i)osition  of  Api)endicularia  wiiich   may 
perhaps  be  nearer  the  truth  than  either  (,f  the  views  mentioned  in 
the    text.      It    IS    not    absolutely   necessary   to   suppose   that   the 
ancestors    of    Appendicularia    were    fixed    Ascidians ;    but    both 
Appendicularia  and  the  fixed  Ascidians  may  have  <lescende<l  from 
a  common    free-swimming   stock,   ami    have    undergone    certain 
modifications  in  common,  such  as  loss  of  true  vascular  svstem  and 
cu."lom.     Then,  while  the  Ascidians  proper  bec;ame  adapted  to  a 
sessile  existence,  Appendicularia  may  be  supposed  to  have  gone 
to  the  opposite  extreme,  and  have  become  adapted  to  an  absolutely 
pelagic  existence.     In  becoming  adapted  to  such  a  jjurelv  pehunc 
or  oceanic  environment  as  that  of  Appendicularia.  it  is  emineiuly 
conceivable  that  an  animal  would  have  to  undergo  as  radical  a 
modification  of  structure  as  it  would  in  becoming  adapted  to  a 
sessile  existence.     (Compare  Sa/pa,  Dolioliim,  etc.) 


■.3i 


V. 


THE  PROTOCHORDATA  IN  THEIR  RELATION 
TO  THE  PROBLEM  OF  VERTEBRATE  DE- 
SCENT. 


\WM'': 


"  Den  Schl'ussel  richtigen  Verstiiudnisses  gibt  nicht  das  Hiueinpressen 
neucr  Thatsaclien  in  cine  altc  Sc/iablone,  sondcrn  dirs  Aufsitchen  dcs 
gendischen  Zusammcnhangs  dcr  Erscheinungen."^  —  VVeismaxx. 

BALaNOGLOSSUS. 
External  Features. 

Of  the  free  living  protochordates,  the  lowest  type  of 
organisation  is  undoubtedly  presented  by  the  Enteropnensta 
(Hemichorda),  the  group  to  which  Balanoglossjis  belongs. 

Balanoglossus  is  a  remarkahie  worm-like  creature  which 
lives  buried  in  the  sand  or  mud  of  th'^  sea-shore.  By 
means  of  numerou  :  unicellular  integumentary  glands  which 
are  distributed  over  the  surface  of  the  body,  it  secretes  a 
mucous  substance  to  which  particles  of  sand  adhere,  and 
so  makes  for  itself  tubes  of  sand  in  which  it  lives  at  about 
the  level  of  the  low  tide-mark.  It  possesses  such  a 
characteristic  external  form  and  odopr(like  iodoform)  as  to 
render  it  peculiarly  easy  of  recognition. 

In  front  there  is  a  long  and  extremely  sensitive /r^/^^^j-m 
which  is  capable  of  great  contraction  and  extension,  and  is, 
in  the  living  animal,  of  a  brilliant  yellow  or  orange  colour. 
Behind  the  proboscis  follows  a  well-marked  collar-region, 

242 


BALANOGLOSSUS. 


243 


ION 
DE- 


bressen 
hen  dcs 


vpe 


re 


of 
ncnsta 
ongs. 
which 
By 
which 
retes  a 
,  and 
about 
such    a 
n)  as  to 

vohoscis 
and  is, 
colour. 
-region. 


consisting  externally  of  a  collar-like  expansion  of  the 
integument,  with  free  anterior  and  posterior  margins  over- 
lapping the  base  of  the  proboscis  in  front  and  the  anterior 
portion  of  the  gill-slits  behind. 

In  the  ventral  middle  line,  at  the  base  of  the  proboscis 
and  concealed  by  the  collar,  is  situated  the  mouth  (Fig. 
113).  Following  behind  the  collar  is  the  region  of  the 
trunk  or  body  proper,  which,  in  the  adult  of  some  species, 
reaches  a  relatively  enormous  length,   even  extending  to 


Fig.  113.  —  Larva  of  Halanoglosms  Kowahvskii,  with  five  pairs  of  gill-slits; 
from  the  right  side.     (After  Baif.son.) 

a.  Anus.  a. p.  Temporary  pedicle  of  attachment,  c.  Collar,  ch.  Notochord. 
,.;'..r.  Gill-slits.     ;;/.  Mouth,    pr.  F'roboscis. 

two  or  three  feet.  The  ectodermal  covering  of  the  body 
consists  in  general  of  ciliated  cells,  among  which  are  scat- 
tered unicellular  mucous  glands  ;  the  cilia,  however,  appear 
to  be  more  prominent  on  the  proboscis  than  elsewhere. 

In  the  region  of  the  trunk,  which  immediately  follows 
upon  the  collar  region,  there  are  a  great  number  of  paired 


244 


THE  PR OrO CHORDA TA . 


f'Jf'l 


P  >■- 


P.^-  '■ 


'-•J 


openings  on  the  dorsal  side  of  the  body,  placing  the  anterior 
portion  of  the  digestive  tract  in  communication  with  the 
outer  world.  These  are  the  gill-slits,  and  they  are  arranged 
strictly  in  consecutive  or  metameric  pairs  to  the  number  of 
upwards  of  fifty  in  the  adult.  In  their  structure,  and  more 
especially  in  the  possession  of  tongue-bars,  they  bear  a 
remarkable  resemblance  to  the  gill-slits  of  Amphioxus. 
This  is  particularly  striking  in  young  individuals.  As  the 
adult  form  is  approached  in  the  development,  the  bulk  of 
the  gill-slits  sinks  below  the  surface,  only  opening  at  the 
latter  by  small  slit-like  pores,  and  thus  their  true  character 
is  obscured  in  a  superficial  view. 

Projecting  into  the  interior  of  the  proboscis  is  a  rod-like 
structure  which  arises  as  an  outgrowth  from  the  alimentary 
canal  dorsal  to  the  mouth.  The  lumen  of  this  endodermic 
diverticulum  becomes  narrowed  down  and,  in  fact,  partially 
obliterated,  while  the  cells  constituting  its  walls  give  rise 
to  a  spongy  vacuolar  tissue  which  strongly  resembles  the 
notochordal  tissue  of  Amphioxus  and  the  higher  Verte- 
brates. On  account  of  its  dorsal  position  above  the  mouth, 
its  endodermic  origin,  and  the  vacnolisation  of  its  cells,  this 
structure  was  identified  by  Bateson  in  1885  as  the  noto- 

chord. 

N'ervous  System  and  Gonads. 

The  nervous  system  of  Balanoglossus  presents  many 
features  of  the  utmost  interest  and  suggestiveness.  It 
consists  essentially  of  an  ectodermal  network  of  nerve-fibres 
forming  the  inner  layer  of  the  skin  (ectoderm)  all  over  the 
body.  In  this  primitive  nervous  sheath,  which  envelops 
the  whole  body,  there  are  certain  definite  local  thickenings. 
Two  of  these  thickenings  occur  respectively  along  the 
whole  length  of  the  dorsal  and  ventral  middle  lines  ii;  the 
trunk-region,  thus  producing  the  dorsal  and  ventral  median 


mm 

L    ''  III  ,*  '  i...         .( 


BALANOGL  OSSUS. 


245 


^nor 
I  the 
nged 
er  of 
more 
ear  a 
oxus. 
.s  the 
ilk  of 
it  the 
racter 

3d-like 
entary 
dermic 
irtially 
ve  rise 
les  the 
Verte- 
mouth, 
lis,  this 
e  noto- 


5  many 
iss.      It 
;e-fibres 
)ver  the 
;nvelops 
enings. 
ng  the 
s  ii;  the 
median 


longitudinal  nerve-cords.  In  the  region  of  the  collar  the 
dorsal  nerve-cord  becomes  entirely  separated  from  the 
ectoderm,  and  this  portion  of  it  contains,  at  least  in  young 
individuals,  a  central  canal  which,  from  its  origin  and 
relations,  was  shown  by  Bateson,  and  more  recently  by 
Morgan,  to  be  homologous  with  the  central  canal  of  the 
vertebrate  spinal  cord.  Anteriorly  the  dorsal  nerve-cord 
becomes  continuous  with  a  specially  dense  tract  of  the 
general  nerve-plexus  at  the  inner  posterior  surface  of  the 


Fig.  114.  —  Diagram  of  the  org^inisation  of  Ralanoqlossiis,  fiom  the  left  side. 
(From  a  drawinij  kindly  lent  by  Professor  T.  H.  MoRdAN.) 

ill.  Alimentary  canal,  bc^.  Coelom  of  proboscis  (anterior  or  pr;Toral  iiodv- 
cavity).  be-.  Coelom  of  collar,  hc^.  Coelom  of  trunk,  b.v.  Blood-vessel,  proceed- 
ing from  the  so-called  heart  (which  lies  at  base  of  proboscis  above  the  noto- 
chord)  to  the  ventral  blood-vessel,  ch.  Xotochord.  covt.  Coinmissure,  between 
dorsal  and  ventral  nerve-cords,  dn.  Dorsal  nerve-cord,  separated  from  the  integu- 
ment in  the  collar-region.  d.b.zK  Dorsal  blood-vessel.  f[l.  Proboscis-gland ; 
modified  ccelomic  epithelium  surrounding  heart  and  front  end  of  notochord. 
m.  Mouth,  p.v.  Pulsating  vesicle,  lying  inside  the  "  heart."  v.h.v.  Ventral  blood- 
vessel,   v.n.  Ventral  nerve-cord. 

proboscis  (Fig.  114).  This  proboscidian  plexus  thins  out 
somewhat  towards  the  anterior  extremity,  but  nevertheless 
forms  a  complete  nerve-sheath  for  the  proboscis  and  indi- 
cates the  sensitive  character  of  the  latter  (Fig.  1 15). 

The  ventral  nerve-cord  does  not  extend  into  the  region 
of  the  collar,  but  from  the  point  where  the  collar  joins  on 
to  the  trunk  the  ventral  cord  is  connected  with  the  dorsal 
nerve-cord  by  a  commissure-like  thickening  of  the  integu- 
mentary plexus,  which  passes  in  the  skin  on  each  side 
round  the  hinder  end  of  the  collar-region  (Fig.  1 14). 


.U~- 


mm'^^ 


246 


T//£  PA'  O  TO  CIIQRDA  TA. 


«:  i 


m. 


il 


I 
II' 


Fig.  115.  —  Diagrammatic  transverse  sec- 
tion through  hinder  region  of  proboscis  of 
Balanoglossus.  (From  a  drawing  kindly 
lent  by  Professor  T.  H.  MORGAN.) 

D.  Dorsal.  V.  Ventral,  bc"^.  Proboscis- 
cavity,  almost  filled  up  by  mesenchymatous 
and  muscular  tissue*  proliferated  from  the 
original  coelomic  Lpithelial  layer  (indicated 
by  the  black  line  below  the  ectoderm). 
p.v.  Pulsating  vesicle,  h.  Heart,  ch.  Noto- 
chord.    n.s.  Integumentary  nerve-plexus. 


The  genital  organs, 
testes  or  ovaries,  accord- 
ing to  the  sex  of  the 
individual,  occur  as  a 
paired  metameric  series 
of  pouch-like  bodies  or 
gonadic  sacs  which  ex- 
tend backwards  far  be- 
yond the  region  of  the 
gill-slits.  The  gonadic 
sacs  are  suspended  in  the 
body-cavity  by  solid  cords 
attached  to  the  dorsal 
integument,  which  be- 
come perforated  in  the 
spawning  season  to  ad- 
mit of  the  expulsion  of  the 
reproductive  elements. 


Metamcj'ism. 

Although  there  is  no  muscular  metamerism  in  Balano- 
glossus, yet  we  have  seen  that  other  organs  (gill-slits  and 
gonads)  are  arranged  metamerically.  And  in  point  of 
fact,  among  those  Invertebrates  which  are  not  included 
under  the  phylum  of  the  Articulata,  if  there  is  one  pecu- 
liarity of  organisation  more  sporadic  in  its  occurrence  than 
another,  it  is  metamerism.  It  may  affect  the  most  differ- 
ent organs  of  the  body  either  collectively  or  individually, 
and  nothing  is  more  patent  than  the  fact  that  the  meta- 
meric repetition  of  parts  has  arisen  independently  over 
and  over  again  in  different  groups  of  animals.^ 

*  This  tissue  is  not  represented  in  Figs.  114  and  116,  although  it  is  present 
throughout  the  body- cavity. 


BAL.-lA'OGLOSSfS. 


247 


Far  from  assuming  as  a  self-evident  fact  that  the 
extreme  metamerism  of  the  Annelids  and  Arthropods  is 
genetically  identical  with  that  of  the  Vertebrates,  we  have 
every  reason  to  suppose  that  it  has  been  elaborated  entirely 
independently  in  the  two  cases,  and  that  the  apparent  simi- 
larity is  due,  as  already  intimated,  to  a  />am//e-/  evolution. 


Body-cavities  ;  Proboscis-pore ;  Collar-pores. 

Corresponding      to      the 

three    regions    into    which 

the  body  of  Balanoglossus  is 

divided,  —  namely,    probos- 
cis, collar,  and  trunk,  —  the 

body-cavity  is  divided  up  into 

three    systems    of   cavities. 

These  are   (a)  the  anterior 

body-cavity  or  cavity  of  the 

proboscis,  (/3)  a  pair  of  collar- 
cavities,  and   (7)   a   pair   of 

body-cavities  which  form  the 

unsegmented  coelom  of  the 

trunk  (Figs.  114,  115). 

These  cavities  arise  essen- 
tially as  pouches  from  the 
archenteron  (Fig.  117),  al- 
though their  actual  develop- 
ment differs  considerably  in 
different  species  (Morgan). 
The  proboscis-cavity  is 
placed  in  communication 
with  the  exterior  by  an  open- 
ing   through    the   posterior   'I'.'n.bos 


Fig.  116.  —  Diagram  of  the  organisa- 
tion of  Balanoglossus,  from  Ihedois.il 
side.  (From  a  drawing  kindly  lent  by 
Professor  'I'.  H.  Mokcan.) 

c.p.  Collar-pores.  ^^^).  Gonads,  g.s. 
Gill-slits ;  the  dark  lines  converging  he- 
hind  indicate  the  superficial  pcirtions  of 
the  gill-slits;  below  the  surface  are  seen 
the   free   ends   of   the   toneue-bars.     //. 


igut 
cis-pore.     Other  letters  as  abovi 


248 


THE  PROTOaiORDATA. 


wall  of  the  proboscis  known  as  the  proboscis-pore.  In 
/).  Kozvalcvskii  this  pore  lies  asymmetrically  to  the  left  of 
the  dorsal  middle  line  (Fig.  115),  while  in  B.  Kupfferi  a 
corresponding  opening  occurs  to  the  right  of  the  middle 

line,  so  that  in  this  species 
there  are  two  proboscis- 
pores  constituting  a  sym- 
metrical pair. 

The  left  proboscis-pore 
of  Balanoglossus  is  obvi- 
ously to  be  compared  with 
the  proeoral  pit  of  Amphi- 
oxus. 

The  collar-cavities  also 
open  to  the  exterior  by 
pores,  one  on  each  side 
underneath  the  dorsal  pos- 
terior free  fold  of  the 
collar,  and  on  a  level  with 

Fig.    1117.  —  Diagrammatic    horizontal  ,  i  •  c     i.\,        ti      .. 

section  through  an  embryo  of  Halanoglos-  ^hc     OpCUmg    of     the     first 

sus  (typi'   of  the   direct  development),   to  ajU-slit.        Thcse     are     the 

show  the   origin    of    the   body-cavities  as  ' 

arche.iteric  pouches.     (After  b'atkson.)  funncl-shapcd     Collar-pOrcS. 

ap.  Tuft   of   cilia    at    the    apical    pole    Cppx,px,i    states  thit  Wlter 
(indicationofan  apical  plate),   ^.i.  Probos-    ^PJ'-N<^'1-L  Siaies  mat  WatCl 

cis-cavity.    hfl.  Collar-cavities.    cbK  Trunk-    is     taken     in     through     the 
cavities,     cb.  Circular  band  of  cilia. 

collar-pores  into  the  cavity 
of  the  collar  in  order  to  swell  the  latter  up,  so  that  it 
may  serve  as  an  accessory  organ  of  locomotion  in  so  far 
as  an  alternate  inflation  and  collapse  of  the  collar  would 
assist  the  animal  in  its  slow  burrowings  in  the  sand. 


\  :*■ 


BALANOGLOSSUS. 


249 


e/^c  e/.fi-U- 


Alimentary  Canal. 

The  mouth  cannot  be  closed,  as  there  is  no  sphincter 
muscle,  and  accordingly,  as  the  animal  progresses  through 
the  sand,  it  swallows  a  large  quantity  of  the  latter  in 
which  food-particles  (unicellular  organisms,  etc.)  may  also 
be  involved.  As  the  sand  passes  through  the  intestine, 
it  becomes  enveloped  in  the  mucous  secretion  of  the  intes- 
tinal epithelium,  and  is  ejected  through  the  anus  in  a  cord 
of  slime. 

The  alimentary  canal  is  a  straight  tube  between  mouth 
and  anus.  In  its  hinder  portion  it  is  usually  sacculated, 
i.e.  provided  with  paired 
lateral  saccular  dilatations 
comparable  to  the  so-called 
intestinal  eoeea  of  the  Ne- 
mertineworms.  (See  below.) 
In  the  region  of  the  pharynx 
the  lumen  of  the  alimentary 
canal  is  incompletely  divided 
by  lateral  constrictions  into  ^^*= 

.  ,.  Pig.  118.— Transverse  section  through 

two     portions,    an    upper    or    the  gill-region  of  Balanoglossus.     (After 

branehicxl   portion    carrying  ^'^''';^^=''j':> 

_  J       o         al.    Digestive    portion    of    gut.      br. 

the  gill-slits,  and   a  lower  or    'branchial    portion    of   gut.      bci.  Third 

»•     ../V  4.-        /TT-  ox     body-cavity  (trunk  ca.>lom)  ;  this  is  also 

digestive  portion  (Fig.    118).    nearly  obliterated  in  the  adult  by  the  pro- 

The  latter  was  compared  bv  ''^''''^^'O"    «f    mesenchyme    or    -paren- 

^  J  chyme      from    its   walls,      d.fi.c.   Dorsal 

GeGENBAUR*    to    the    endo-  nerve-cord,      d.b.v.   Dorsal  blood-vessel. 

i    1         f   ^1  A       •  !•  ,  i'"'   Cjonad.    p.s.  Gill-slit.     t.b    Tont^ue- 

Style    of   the    AsCldianS,    but  bar.    v.b.v.   Ventral    blood-vessel.     ;«... 

it  is  probable  that  this  com-  '^''"*''''''  "^rve-cord. 

parison,  although  a  very  natural  and  useful  one  at  the  time 

at  which  it  was  made,  will  not  hold  good,  since  there  is 

*  Carl  Gegenbaur,  Elements  of  Comparative  Anatomy.    Translated  by 
F.  Jeffrey  Bell.     London,  1878. 


t^6.iA 


1 


'SO 


THE   PR  0  TO  CHORDA  TA. 


nothing  in  the  structure  or  development  of  this  part  of  the 
alimentary  tract  in  lialanoglossus  which  will  bear  compari- 
son with  the  endostyle.*  As  indicated  in  the  larvic  of 
Amphioxus  and  the  Ascidians,  it  would  seem  that  the 
endostyle  first  became  evolved  or  differentiated  at  the 
anterior  end  of  the  pharynx,  ///  front  of  the  gill-slits,  in 
correlation  with  the  dorsal  position  of  the  mouth. 


^ 


Dcveloptncnt ;   the   Tornaria  Larva. 

The  development  of  Balatwglossns  Koivalcvskii  as  made 
known  to  us  by  the  admirable  work  of  Bateson  is  what 
is  known  as  a  strictly  direct  devclopvicnt ;  that  is  to  say,  the 
embryonic,  larval,  and  adult  stages  follow  one  another  by 
gradual  transitions  concomitantly  with  the  simple  progres- 
sive growth  of  the  individual  and  without  any  striking 
mctanwrpliosis.  In  other  species  of  Balanoglossus  the 
larval  form  is  remarkably  different  from  the  adult,  and 
becomes  transformed  into  the  latter  by  a  very  distinct 
metamorphosis.  The  extraordinary  larval  form  here  re- 
ferred to  was  discovered  in  1848  by  Johannes  Muller, 
who  named  it  Tornaria,  and  regarded  it,  as  did  his  succes- 
sors Krohn,  Alexander  Agassiz,  and  Fritz  Muller,  as 
the  larva  of  an  I^chinoderm  (Starfish), 

It  was  not  until  1869  that  its  true  character  as  the  larva 

*  A  ciliated  tract  in  the  floor  of  the  (vsophagus  of  a  Tornaria  from  the 
Pacific  has  recently  been  comparetl  to  the  eiulustyle  by  W.  E.  Rrn'KR.  ((-*« 
a  A'ew  Balanoglossus  Larva  from  the  Coast  of  Califoniia  and  its  Possession 
of  an  Endostyle.     Zool.  Anz.  XVII.     1894,     jip.  24-30.) 

The  comparison  is  at  present  somewhol  doubtful.  More  recently  GAUbTANt; 
has  suf^jjested  that  the  endostyle  is  derived  from  the  adoral  ciliated  band  of  the 
Kchinoderni  larva.  (See  Fig.  119.)  Th.'  sue;<:;estion  is  an  interesting  one,  but 
(!arstang"s  idea  of  the  relations  of  the  pnn)ral  lobe  is  very  different  to  the  one 
here  set  forth.  (Wai.TKK  CIaksianc,  Preliminary  Xote  on  a  New  Theory  of 
the  Phytogeny  of  the  Chordata.     Zool.  .\n/.  XVII.     pp.  122-125.) 


1 


■'■'  •♦: 


T 


B.IAAA'OGLOSSC/S. 


251 


of  a  species  of  BrJ.anoglossus  was  demonstrated  by  Elias 
Metschnikoff.  Shortly  afterwards,  Metschnikoff's  dis- 
covery was  confirmed  and  amplified  by  Alkxanuer 
Agassiz. 

The  superficial  likeness  between  Tornaria  and  such  I^chi- 
noderm  larvx^  as  Bipinnaria  or  Auricularia  is  astonishini;-, 
and  a  renewed  study  of  the  detailed  organisation  of 
Tornaria,  recently  made  by  Morgan,  appears  to  have 
established  the  fact,  originally  insisted  upon  by  Metschni- 
koff, that  this  resemblance  can  only  be  accounted  for  on 
the  ground  of  genetic  affinity. 

In  Figs.  119  and  120  two  types  of  larvne,  Tornaria 
and  Auricularia,  are  shown  side  by  side  ;  and  although 
unfortunately  they  are  not  figured  from  exactly  the  same 
aspect,  yet  it  is  obvious  at  a  glance  that,  in  spite  of  certain 
differences  which  will  be  enumerated  below,  they  both 
belong  to  the  same  category  of  larval  forms. 

A  highly  characteristic  feature  of  these  larvae  is  the 
remarkable  ectodermal  ciliated  band  which  constitutes  a 
perfectly  symmetrical  but  somewhat  complicated  undulat- 
ing seam  round  the  body.  The  larvae  are  strictly  pelagic, 
and  swim  about  in  the  open  sea  by  means  of  their  cilia ; 
but  the  latter,  instead  of  being  distributed  evenly  over  the 
whole  surface  of  the  body,  are  concentrated  in  the  region 
of  the  ciliated  bands  which  are  composed  of  thickened 
ectoderm. 

In  Tornaria  there  are  two  ciliated  bands,  viz.:  i)  the 
above-mentioned  undulating  seam  which  is  usually  known 
as  the  circinnoral  or  longitudinal  ciliated  band,  and  2)  a 
pastoral  circular  ciliated  band.  Only  the  former  is  present 
in  Auricularia,  and  the  absence  of  the  circular  band  in  this 
form  constitutes  one  of  the  chief  differences  between  the 
two  larvce. 


*M 


252 


THE  PKOTOCIIORDA  TA. 


w 


From  a  morphological  point  of  view  a  more  striking 
resemblance  between  the  two  larvae  than  that  furnished 
by  the  longitudinal  ciliated  bands  exists  in  connexion  with 
the  anterior  body-cavity  or  cntcroaxl.     In  the  Echinoderm 


HP-- 


rn  — -'i 


Figs.  119  and  120.  —  Auricularia,  larva  of  Synapta  (after  Semon)  ;  and 
Tornaria,  larva  of  lialanoglossus.     (After  MoRii.vN.) 

a.  Anus.  a.p.  -Apical  plate,  bc^.  Anterior  body-cavity,  communicating  witii 
exterior  by  the  water-pore,  be-,  bc^.  Second  and  third  body-cavities  of  Tornaria. 
c.b.  Circular  ciliated  liand  of  Tornaria.  c.c.  Contractile  cord  between  apical  jilate 
and  anterior  body-cavity  of  Tornaria.  g.p.  Gill-pouches.  Ii.c.  Hydrocoel  of 
Auricularia  (anterior  body-cavity),  l.c.b.  Longitudinal  (circumoral)  ciliated  band. 
I.e.  Left  enterocoel  (body-cavity),  m.  Mouth.  «.  Lateral  (paired)  nerve-band 
of  Auricularia.  r.e.  Right  enterocoel.  sp.  Calcareous  spicules,  st.  Stomach. 
•u>p.  Water-pore. 

N.B. —  In  Auricularia,  the  margin  of  the  mouth  is  surrounded  by  a  ciliated 
band  discovered  l)y  Skmon,  and  known  .as  the  adoml  ciliated  band.  The  poste- 
rior, V-shaped  portion  of  this  band  lies  inside  on  the  ventral  floor  of  the  larval 
oesophagus. 


larva  this  cavity  arises  as  a  median  pouch  of  the  arche.  - 
teron,  and  there  is  every  reason  to  suppose  that  it  has  a 
similar  origin  in  Tornaria,  although  this  point  has  not  yet 


BALAXOGLOSSC'S. 


253 


-6c' 


•C.L 


iK-en  cletcrmined.  The  primary  anterior  enterocoel  in  the 
I'Lchinoderm  larva  is  not  quite  the  same  as  the  correspond- 
ing cavity  in  Tornaria,  since  it  contains  also  the  elements 
of  the  general  body-cavity.  Apart  from  slight  differences, 
the  collar-cavities  and  general  body-cavities  arise  essen- 
tially in  the  same  way  in  Tornaria  as  they  do  in  the  case 
of  the  direct  developing  larva  of  J^alanoglossus  (see  above).* 
In  the  Echinoderm  larva,  however,  the  paired  body- 
cavities  do  not  arise  as  independent  archcnteric  pouches, 
but  they  become  constricted  off  from  the  anterior  entero- 
cad.  Making  allowance  for  these  deviations  in  the  origin 
of  the  body-cavities,  —  deviations  which  are  by  no  means 
fundamental,  since  in  both  cases  the  body-cavities  are 
ultimately  reducible  to  archenteric  pouches,  —  it  is  an 
extremely  striking  fact  that  bc^th  in  Tornaria  and  Auricu- 
laria  the  anterior  enterocoel  acquires  an  opening  to  the 
exterior  on  the  dorsal  surface  to  the  left  of  the  middle  line. 
This  opening  is  called  the  i^'ater-pore,  since  it  forms  the 
outlet  (possibly  both  outlet  and  inlet)  of  the  water-vascular 
system  of  the  Echinoderm.  In  Tornaria  it  persists  after 
the  metamorphosis  as  the  proboscis-pore,  which  has  been 
described  above. 


The  Larva  of  Asterias  vuli^aris ;    Water-pores  and 
Precoral  Lobe. 

In  view  of  what  was  said  above  as  to  the  occurrence  of 
paired  proboscis-pores  in  B.  Kiipfferi,  it  is  interesting  to 
note  that  sometimes  there  are  two  water-pores,  a  right  and 
a  left,  in  Echinoderm  larva}.     This  has  been  observed  by 

*  As  to  the  origin  of  the  body-cavities  in  different  species  of  Balanoglos- 
sus,  Morgan  summarises  his  observations  as  follows:  "They  may  arise  as 
enteric  diverticula,  as  endodermal  proliferations,  or  even  arise  from  mesenchy- 
matous  beginnings."     (See  Morgan.     No.  125  bijjliog.) 


254 


THE  PROTOCHORDATK 


1 

fflra;' 

tlfc'^ 

W- 

t 

Jci 


oe^ 


Bkooks  and  G.  W.  Field  in  the  larvnc  of  a  common  star- 
fish, Asteriixs  viilij^iwis.  In  this  case  the  primary  enterocoel 
becomes  constricted  off  from  the  archenteron  in  the  form 
of  two  equal  pouches.  The  ri<,^ht  and  left  enteroccelic  sacs 
then  take  up  a  symmetrical  position  on  each  side  of  the 
larval  oesophagus,  and  each  sac  next  opens  to  the  exterior 
by  a  ivatcr-porc.  The  pore  in  connexion  with  the  right 
sac  (Fig.   121 )  is,  however,  of  a  transitory,  rudimentary 

character,  and  soon  closes 
up,  while  the  left  pore  per- 
sists as  the  definitive  water- 
pore.  As  in  Tornaria,  so 
here,  the  cavity  of  the  larval 
body  generally,  and  of  the 
prncoral  region  {praoral  lobe) 
in  particular,  is  the  primary 
body-cavity  or  blastococl, 
and  contains  scattered  mes- 
enchyme-cells.     At  a  later 

Young  larva  of  Asterias  Stage   in   the   larva  of   As- 
vuigarn,  from  the  dorsal  side.    (After  terias    the    richt    and    left 

G.  W.  FlKl.D.)  ,  •= 

p.i.  Fraorai  lobe,    i.c.b.  Circumorai  cnterocoelic  sacs,  having  in- 

(longitudinal)  ciliated  band.   ««.  tp'.soiih-  ,  ^i        •        i  ^i 

agus     r.e.  and  /...  Right  and  left  en-  crcascd   greatly   m    length, 

teroca;lic  sacs,  each  opening  by  a  "  water-    ^ncet      OnC     another    in     the 

pore"  to  the  exterior.    jA  Stomach.    /;//. 

Aperture,  leading  from  stomach  into  in-    region    of    the    prncoral    lobc 

and  fuse  together,  thus  put- 
ting their  two  cavities  into  communication  across  the 
median  line.  The  median  portion  of  the  enterocoel  thus 
produced  extends  up  into  the  prceoral  lobe,  and  so  the 
primary  blastocoelic  cavity  of  the  latter  is  replaced  by  a 
secondary  ingrowth  of  the  enterocoel  (Fig.  122). 

Similarly    with    the    metamorphosis    of    Tornaria,    the 
anterior    enterocoel,    which    is   at   first   of   very  inconsid- 


Fig.   121. 


B.tl.AXOG/.OSSCS. 


255 


erable  extent  (Fig.  120),  increases  greatly  in  size,  and 
assumes  its  definite  position  and  proportions  as  the  cavity 
of  the  praioral  lobe  {i.e.  proboscis),  thus  replacing  the 
original  blastocctlic  space, 
while  the  water-pore  remains 
as  the  probosris-pore. 

As  described  in  the  previ- 
ous chapter,  the  ^avity  of 
the  prccoral  lobe  (fixing 
stolon)  of  the  Ascidian  tad- 
pole is  of  the  nature  of  a 
blastoccel  or  primary  body- 
cavity,  containing  loose  mes- 
enchyme-cell.s,     nncl     it     is 

therefore      of     great     impor-  F;g-  ^".-Olcler  larva  (Hipinnan., 

°  »  ot    .Is/c'r/iis    x'uii^'ans,    from    the    ventral 

tance  to  note  that  whether  side.    (Aft,i  <;.  w.  fiki.d.) 

•  r       1  1  '^y  ^  fusion  of  the  two  pra,'oral  loojis 

the     cavity     of     the     prrCOral    of  tlie  ciliated  hand  across  the  apex  of  the 

lobe     is     a     blaStOCal    or      an    F^'O-allobe.  followed  by  a  separation  in 

the   transverse    direction,    the    ons,'inally 

entcrOCCCl,  the  morphological    s'lR't'  circumoral  band  (cf.  Fi.s^s.  119  and 

121)  has  become  chvided  into  two  l)ands. 


value  of  the  structure  itself 
remains  the  same. 


a  pr.'i'oral  ciliated  band  p.c.b.  and  a  post- 
oral  longitudinal  ciliated  band  l.c.b.  The 
posterior  transverse  portion  of  the  pr;i'- 
oral  ciliated  band  has  under>;one  a  fusion 
with  the  front  end  of  the  originally  dis- 
tinct adoral  band  (cf.  Fig.  119).  p.l.  Vx?c- 
oral  lobe,  into  which  the  enterocoel  has 
extended,  m.  Moutii.  r.e.  and  I.e.  Right 
and  left  cnterocaulic  cavities  st.  Stomach, 
a.  Anus. 


Apical  Plate  of  Tornaria. 

At  the  anterior  end  of 
the  body,  or,  in  other  words, 
at  the  apex  of  the  prrcoral 
lobe,  in  Tornaria,  there  is  an  ectodermic  thickening  in 
which  nerve-cells  and  nerve-fibres  and  a  pair  of  simj^le 
eyes  have  become  differentiated.  This  is  the  so-called 
apical  plate,  and  it  constitutes  the  central  nervous  .system 
of  the  larva.  It  can  be  recognised  for  some  time  after  the 
metamorphosis  at  the  tip  of  the  proboscis,  but  eventually 
disappears  completely.     A  similar  apical  plate  occurs  in 


H. 


ip?:f««ip 


M 


1^'- 


r9 


2  $6 


THE  PROTOCHORDA  TA. 


litt: 


I 
I 


I 


m. 


i 


a  great  number  of  Invertebrate  larv?e,  and  is  especially 
characteristic  of  the  free-swimming  larvrc  (Trochophores, 
or  Trochospheres)  of  Annelids  and  Molluscs.  We  shall 
return  to  thi;;  later. 

In  Tornaria  a  single  contractile  cord  passes  from  the 
apical  plate  to  the  anterior  enteroccel. 

There  is  no  apical  i)late  in  Auricularia,  nor  in  most  of 
the  other  I-.chinoderm  lai-vi\: ;  but  there  is  reason  to  sup- 
l)ose  that  it  has  been  secondarily  lost,  since  a  transitory 
ectodermal  thickening  at  the  apical  pole  can  frequently 
be  observed  in  the  course  of  their  development  ;  and, 
moreover,  in  what  is  probably  the  most  primitive  Echino- 
dcrm  larva  known  (viz.  the  larva  of  the  Crinoid,  Anfcdon), 
there  is  a  well-developed  apical  plate. 

Mctauiorphosis  of  Toniaria. 

The  metamorphosis  of  Tornaria,  as  originally  described 
by  Alexander  Agassiz,  takes  place  with  relative  sudden- 
ness. According  to  the  more  recent  account  of  the  meta- 
morphosis  given  by  Morgan,  a  marked  diminution  in  size 
occur!-  ;  the  internal  organs  are  drawn  together  in  such  a 
way  that  the  larval  oesojihagus,  with  the  gill-pouches  (see 
Fig.  I2u),  is  drawn  backwards  into  the  body,  and  the 
anterior  enceroccel,  as  already  described,  is  carried  for- 
wards into  the  prxoral  lobe.  The  longitudinal  (circum- 
oral)  ciliated  band,  which  was  the  first  to  develop,  is  also 
the  first  to  disappear,  while  the  posterior  circular  band 
persists  t'.)  a  "somewhat  later  stage. 

The  Ncmcrtincs. 

It  is  thus  evident  that  Balanc  glossus,  especially  through 
its  Tornaria  larva,  shows   undoubted  marks  of  affinity  to 


^'EME/^77XES. 


2S7 


of 


the  Echinoderms.  It  will  next  be  shown  that  there  are 
certain  features  in  the  adult  anatomy  which  apparently 
indicate  a  distinct  genetic  relationship  to  another  group  of 
the  Invertebrates  ;  namely,  the  iVcwcrtine  worms.  ' 

The  Nemertines  are  elongated,  flaticned,  or  cylindrical 
worms,  with  a  smooth  cihafai  skin  and  no  external  seg- 
mentation, occurring,  as  a  rule,  in  a  closely  similar  habitat 
to  that  of  Balanoglossus,  buried  in  the  sand  or  mud  of  the 
sea-shore. 

Like  Balanoglossus,  they  also  possess  uriccllular  intcni- 
mcntary  glands,  by  means  of  which  they  .ecrete  a  muco'lis 
substance,  to  which  frequently  sand-grains  adhere,  thus 
producing  a  tube  of  sand  round  the  body.  Some  of  them 
reach  an  enormous  length,  and  one  at  least  must  be 
measured  m  yards  {Uncus  longissimns  exceeding  three 
yards  in  length). 

The  chief  anatomical  features  which  offer  material  for 
direct  comparison    between  the  Nemertines  and    Balano- 
glossus relate  to  the  ectoderm,  proboscis,  nervous  system 
mesenchymatous  tissue,  the  reproductive  organs,  and  the 
alimentary  canal. 

As  for  the  ectoderm,  considered  apart  from  the  nervous 
system,  it  need  only  be  repeated  that  in  both  cases  it  is 
composed  of  ciliated  cells  and  scattered  mucous  glands 

The  proboscis  of  the  Nemertines  is  one  of  "the  most 
characteristic  organs  of  this  group  of  animals.  It  is  not 
permanently  protruded,  and  does  not  serve  as  an  oro-an  of 
locomotion,  as  in  Balanoglossus,  but  is  usually  c'lrricd 
about  entirely  withdrawn  within  the  body  of  the  animal, 
from  which  it  can  be  shot  out  with  great  force  and  rapidity 
when  the  occasion  demands  it.  During  the  process  of 
extrusion  it  is  turned  completely  inside  out,  and  conversely, 
during  the   process   of   introversion,  the  retraction    takes 


mmt 


ii 


258 


77/E   rKOTOCHORDATA. 


place  from  the  tip  backwards  by  the  in-rolling  of  its  walls. 
According  to  the  graphic  description  of  Hubrecht,  it  is 
retracted  "in  the  same  way  as  the  tip  of  a  glove  finger 
would  be  if  it  were  pulled  backwards  by  a  thread  situated 
in  the  axis  and  attached  to  the  tip." 

When  at  rest  within  the  body  the  proboscis  lies  freely 
within  a  hollow  cylinder,  the  wall  of  which  is  thick  and 
muscular,  and  constitutes  XhQ  ptvboscis-sheath  (Fig.  123). 


I 


It:'        ;iiSZE 


Fig.  123.  —  DiagramiiKilic  transverse  section  through  the  middle  of  the  body 
of  a  Nomertiiic.     (After  LaNC,  Tfxt-book  of  Coinp.  Avat.) 

I'.iii.  Masenu-nt-niembrane.  cm.  Circular  muscles,  d.n.  Dorsal  or  "  medullary" 
nerve,  d.v.  Dorsal  blood-vessel.  £'.  Gonads,  int.  Intestine.  Aw.  Longitudinal 
muscles,  l.n.  Lateral  nerves,  l.v.  Lateral  blood-vessel,  p.  Proboscis,  p.s.  Pro- 
boscis-sheath. 

Sometimes  beneath  the  ectodermal  epithelium  of  the 
Nemertine  proboscis  there  is  a  continuous  sheath  of  nerve- 
fibres,  comparable  to  the  nervous  plexus  in  the  proboscis 
of  Balanoglossus. 

Partly,  therefore,  on  account  of  its  structure,  and  partly 
on  account  of  its  topographical  relations  when  extruded, 
we   are    led  to    suppose    that    a   certain    homology  exists 


XEMEK  -J'lXES. 


259 


)f   the 
Incrve- 

Iboscis 

partly 

Irudcd, 

exists 


between  the  retractile  proboscis  of  the  Nemertines  and 
the  non-retractile  proboscis  of  Balanoglossus  (Bat'.^on). 

In  the  most  primitive  Nemertines  the  nervous  system 
consists  essentially  of  a  somewhat  complicated  pair  of 
cerebral  ganglia  and  a  diffuse  nerve-plexus,  with  nerve- 
cords  lying  at  the  base  of  the  ectoderm.*  As  the  cerebral 
ganglia  probably  belong  to  the  same  category  as  the  cere- 
bral ganglia  of  all  other  typical  Invertebrates,  and  are  not 
represented  in  Balanoglossus,  we  can  afford  to  neglect 
them  at  present.  Confining  our  attention  to  the  ecto- 
dermal nerve-plexus,  we  find  occurring  in  it,  along  definite 
lines,  local  thickenings,  after  the  same  principle,  but  not 
all  on  the  same  lines,  as  was  described  above  for  I^alano- 
glossus.  Directly  comparable  wdth  the  dorsal  longitudinal 
nerve-cord  of  Balanoglossus,  there  ii,  a  similar  thickening 
or  concentration  of  the  integumentary  nerve-plexus  in 
some  of  the  Nemertines,  in  the  dorsal  middle  line  {Car- 
i)ii}ia,  Ccphalothrix).  Hubrecht,  who  discovered  this,  calls 
it  the  medullary  nerve.  There  is,  however,  no  correspond- 
ing ventral  nerve-cord  in  the  Nemertines,  but,  instead  of 
this,  there  is  a  pair  of  lateral  thickenings,  constituting  the 
well-known  lateral  nerves  of  the  Nemertines  (Fig.  124). 

It  is  usually  supposed  that  the  lateral  nerves  of  the 
Nemertines  are  homologous  with  the  two  halves  of  the  ven- 
tral nerve-cord  in  the  Annelids.  In  the  Annelids  the 
primitive  lateral  nerves  (which  are  so  typical  of  the  Platy- 
helminths,  or  flat-worms)  have  approached  one  another  in 
the  mid-ventral  line,  and  have  often  undergone  intimate 
fusion  together.  In  some  cases,  however,  they  are  separated 
from    one   another  by  a  wide   interval  (Sabellaria,   etc.). 

*  HuHRKCliT  compared  the  lobes  of  the  cerebral  ganglia  of  a  Xeniertine  to 
the  cranial  ganglia  of  the  Vertebrates,  the  lateral  nerves  to  the  Rami  laterales 
vagi,  and  the  jirolioscis-sheath  to  the  notochord. 


if 

I: 


260 


7'//£   PROTOCHOKDATA. 


In  the  Annelids,  in  contrast  to  the  Nemertines,  the  gan- 
glion-cells are  not  distributed  uniformly  along  the  whole 
length  of  the  nerve-cord,  but  are  collected  together  to 
form  definite  ganglionic  swellings. 

It  is,  therefore,  very  significant  that  in  the  Nemertines 
we  have  a  median  dorsal  "medullary"  nerve,  in  addition 
to  the  elements  which  constitute  the  ventral  nerve-cord  of 
the  Annelids, 

In  many  Nemertines  the  dorsal  and  lateral  nerve-cords 
do  not  continue  to  lie  in  the  ectoderm  throughout  life,  but 


dn 


i 


Fig.  124.  —  Diagrammatic  view  of  anterior  portion  of  a  Nemertine,  from  the 
left  side.     (After  HUHRKCHT,  from  I.-ANC.) 

a.l.  Anterior  lobe  of  brain.  /./.  Posterior  lobe  of  brain.  /.  Opening  of  pro- 
boscis.    VI.  Mouth,    d.n.  Dorsal  nerve,     l.n.  Lateral  nerve      r.n.  Ring-nerves. 

sink  deeper  into  the  body,  and  so  come  to  be  separated 
from  the  ectoderm,  first  by  the  basement  membrane,  and 
then  by  one  or  more  muscular  layers  of  the  body-wall.  In 
the  Hoplonemertea  (those  in  which  the  proboscis  is  armed 
with  stylets)  the  medullary  nerve  is  absent.  In  all  cases, 
however,  the  longitudinal  nerve-cords  remain  connected 
with  one  another  by  a  more  or  less  plexiform  arrangement 
of  nerve-fibres  ;  although  sometimes  a  more  definite  con 
nexion,  by  means  of  metameric  ring-nerves,  has  been 
observed  by  Hubkecht  (Fig.    124). 


«&■ 


NEMERTINES. 


261 


There  is  no  true  coelom  in  the  Nemertines,  and  the 
space  between  the  alimentary  canal  and  body-wall  is  oc- 
cupied by  a  gelatinous  mesenchyme,  containing  muscular 
and  connective  tissue  elements.  In  Balanoglossus  the  cav- 
ity of  the  coelom  becomes  largely  obliterated  in  the  adult, 
by  the  proliferation  of  cells  from  the  epithelium  of  its 
walls,  thus  filling  up  the  cavities  with  a  more  or  less  solid 
parenchymatous  tissue. 

Like  Balanoglossus,  the  Nemertines  have  a  straight  ali- 
mentary canal,  provided  with  paired  lateral  outgrovvths  or 
intestinal  cara,  and  a  terminal  anus. 

The  gonadic  sacs  of  the  Nemertines  offer  a  striking  re- 
semblance to  those  of  Balanoglossus.  They  occur  "Ls  a 
metameric  series  of  paired  sacs,  which  alternate  with  the 
above-mentioned  intestinal  coica,  and  communicate  with 
the  exterior  by  short  tubes,  which  are  at  first  solid,  as  in 
Balanoglossus,  subsequently  becoming  hollowed  out  and 
opening  above  the  lateral  cords  (P^'ig.  124). 

Finally  it  should  be  rointed  out  that,  while  excretory 
organs,  in  the  form  of  a  well-developed  single  pair  of 
elongated  nephridia,  provided  with  numerous  internal 
"end-sacs,"  are  present  in  the  Nemertines,  nothing  of  the 
kind  has  yet  been  detecte  1  in  Balanoglossus. 


CEPHALODISCUS   A\D    KIIABDOPLEURA. 

It  is  interesting  to  note  that  there  are  some  remarkable 
animals  which  stand  in  a  similar  relation  to  Balanoglossus 
that  the  Ascidians  do  to  Ami)hioxus.  While  Balano- 
glossus is  free-living,  does  not  produce  buds,  and  has  a 
straight  alimentary  canal,  these  creatures,  of  which  only 
two  genera  are  at  jjresent  known,  Ccphalodiscns  and  Rhab- 


doplcura,  lead  a  sessile  exist 


'cphalodiscns  and  Rhab- 
ence.  produce  buds,  and  liave 


m 


L 
if 


262 


r//£   PR O 7 'O CHORD. I'J'A. 


a  U-shaped  alimentary  canal.  Both  are  deep-sea  forms, 
Cephalodiscus  having  been  dredged  during  the  Challenger 
Expedition,  from  the  Straits  of  Magellan,  at  a  depth  of  245 
fathoms  ;  while  Rhabdopleura  was  first  dredged  indepen- 
dently, off  the  Shetland  Islands,  at  90  fathoms,  by  the  Rev. 


m 


.mi 


is 


Fig.    125.  —  Cephalodisctts     diHit'cak'phiis,     from     the    ventral     side.       (After 

M'lMOSlI.) 

Actual  length  of  polypide  from  extremity  of  branchial  plumes  to  the  tip  of  the 
pedicle  is  about  2  mm. 

b.s.    I^ut'cal   shield;    the   shading   on    its   surface   indicates   pigment-markings. 
At  the  tip  of  the  pedicle,  buds  are  produced. 

Canon  Norman,  and  off  the  Lofoten  Islai  ds,  at  2C0  fath- 
oms, by  Professor  G.  O.  Sars  ( 1866-68).  Rhabdopleura  is 
the  name  given  by  Allman  (1869),  who  published  a  short 
account  of  it  ;  and  it  has  since  been  described  by  Saks, 
Lankkster,  and  G.  H.  Fowier. 


5     ^f! 


CEPHALODISCUS. 


263 


The  account  which  we  possess  of  Cephalodiscus  forms 
one  of  the  Challenger  Reports,  and  was  written  by  Pro- 
fessor VV.  C.  M'Intosh,  who  made  out  the  main  features 
of  its  anatomy.  It  was  further  treated,  from  a  morpholog- 
ical standpoint,  by  Sidney  F.  Harmek,  who  pointed  out 
its  remarkably  close  affinity  to  Balanoglossus. 

The  most  important  morphological  features  in  the  anat- 
omy of  Cephalodiscus  are  shown  in  Figs.  125-127.  The 
individuals  live  in  colonies,  in  a  "  house "  or  ojcnocciiiin, 
which  consists  of  a  ramifymg  and  anastomosing  system  of 
tubes,  the  walls  of  which  are  composed  of  a  semi-trans- 
parent, gelatinous  material,  whose  outer  surface  is  covered 
with  spinous  projections.  The  walls  of  the  coenoecium 
are  furthermore  perforated  by  numerous  apertures,  which 
allow  of  the  ingress  and  egress  of  water. 

The  adult  members  of  a  colony  have  no  organic  con- 
nexion between  themselves,  but  each  one  is  independent 
and  free  to  wander  about  the  tunnels  of  the  coenoecium. 
Although  Cephalodiscus  has  not  been  studied  in  the  living 
condition,  there  is  every  reason  to  suppose  that  it  moves 
about  in  its  tube  by  means  of  the  large  buccal  sJiield  ( Fig. 
125)  overhanging  the  mouth,  by  which  it  can  attach  itself 
to  the  inner  surface  of  the  tube,  and  then  help  itself 
along  by  the  curious  pedicle  which  occurs  ventrally  near 
the  hinder  end.  It  thus  seems  probable  that  this  pedicle 
can  be  used  as  a  sucker,  but  its  chief  function  lies  in  the 
production  of  buds  which  grow  out  from  it,  and  eventuallv 
become  detached.  Bateson  has  described  a  somewhat 
similar  sucker  at  the  hinder  end  of  the  body  in  young 
individuals  of  Balanoglossus  (Fig.    113). 

Behind  and  above  the  buccal  shield  there  is  a  row  of 
twelve  tentacles  or  branchial  plumes,  each  possessing  a 
central    stem    or   shaft    which    carries    numerous    lateral 


264 


THE  rROTOCJlORDATA. 


i 


I, 

1 


I 


pinn?e.  An  important  function  of  these  plumes  is  to 
produce  currents  of  water  by  the  action  of  their  ciHa, 
which  vibrate  in  such  a  direction  that  the  water  with 
food-particles  is  letl  into  the  mouth.  The  superfluous 
water  is  led  out  from  the  proximal  portion  of  the  aliment- 
ary canal  by  a  sinj^le  pair  of  gill-slits  which  are  not  visible 

in  surface  view,  since  they 
are  overhung  by  a  fold  of 
the  integument  known  as 
the  post-oral  lamella  or 
opcrciilnui,  corresponding  to 
the  posterior  free  fold  of 
the  collar  in  Balanoglossus 
(Fig.  126). 

In  its  internal  organisa- 
tion, if  due  allowance  be 
made  for  its  U-shaped  ali- 
mentary canal,  Cephalodis- 
cus  greatly  resembles  l^ala- 
noglossus  (Figs.  126,  127). 
The    buccal    shield    of    the 

Fig.  126. -Longitudinal  frontal  (right    f<>'-mcr      is       obvioUsly       the 

an.ii.'ft)sc(tiontino„aha.>nciuitcepi)aio-  equivalent    of    the    probos- 

discus.     (After  Harmfk.)  ' 

hfi.  Second  jiortion  of  body-cavity  cis  of  the  latter,  and  thc 
(collar-conlom").  bfi.  Third  portion  of  .  i  •  U  V  \-  ' 
iiody-cavity  (trunk  ccjulom).  br.  Pharynx.  Cavity  WhlCtl  it  Contains 
c.p.  Collar-pores,  .^.f.  Gill-slits,  int.  In-  corrCSponds  tO  the  probos- 
testine.  n.s.  Nervous  system,  op.  Oper- 
culum. CCS.  LF.sophagus.  st.  Stomach.  cis-CavitV.  MorCOVCr,  thc 
/.  Base  of  tentacle.  ,           .             •,       •       r^       ^     t 

proboscis-cavity  in  Ccphalo- 
discus  {i.e.  the  cavity  of  thc  buccal  shield)  communicates 
with  the  exterior  by  two  proboscis-pores  placed  right  and 
left  of  thc  dorsal  middle  line. 

Following  behind  the  buccal  shield  is  the  collar-region, 
from    which   the    branchial   plumes   arise   dorsally,    while 


CEPHAL  ODISCUS. 


265 


laterally  and  ventrally  it  is  produced  into  a  free  told  to 
form  the  above-mentioned  operculum.  The  collar-region 
contains  a  section  of  the  ccelom  which  is  precisely  homolo- 


Fig.  127.  —  Longitudinal  sagittal  section  through  an  adult  Cephalodiscus. 
(After  Haumkr.) 

'I'lic  section  is  supposed  to  he  taken  sufficiently  to  one  side  of  the  middle  line  to 
allow  of  the  representation  of  one  of  the  ovaries  and  one  of  ttie  proboscis-pores. 

a.  Anus.  b.c.  Trunk-coHom.  ex.  Collar-crelom.  ch.  Notoehord.  ////.  Intes- 
tine. ;;;.  Mouth.  nj.  Nervous  system.  <>/*.  I'ostoral  lamella  (oiierculum). 
ov.  Ovary;  the  oviduct  is  deeply  jjigmented.  p.c.  I'ra'firal  ca;lom  (cavity  of 
buccal  shield),  ph.  I'harynx.  p.p.  I'roboscis-pore.  pcd.  Base  of  pedicle. 
St,  Stomach. 


gous  with  the  collar-cavities  of  Balanoglossus.  As  in  the 
latter  form,  it  communicates  with  the  exterior  by  a  pair 
of   collar-pores  which   open   at   the   level   of   the  i;-ill-slits. 


M^ 


266 


77/ E   PA'  OTO  C//ORDA  TA . 


I 


The  collar-coelom  is  continued  posteriorly  into  the  opercu- 
lum, and  anteriorly  into  the  twelve  tentacular  appendages. 

Finally,  behind  the  collar  comes  the  region  of  the  boily 
containing  the  viscera,  which  are  surrounded  by  the  third 
section  of  the  ccelom. 

Only  the  female  reproductive  organs  have  been  observed 
up  to  the  present  time  in  Cephalodiscus.  They  occur  as 
a  pair  of  gonadic  sacs,  opening  to  the  exterior  on  each 
side  of  the  dorsal  middle  line  between  the  anus  and  the 
central  nervous  system.  The  latter  is  very  simple,  being 
represented  merely  by  a  dorsal  thickening  of  the  ectoderm, 
with  nerve-fibres  in  the  region  of  the  collar  and  posterior 
portion  of  proboscis. 

Finally,  a  short  notochordal  diverticulum  projects  into 
the  base  of  the  buccal  shield  as  in  lialanoglossus. 

Rhabdoplcnra  differs  considerably  from  Cephalodiscus 
in  many  respects,  but,  nevertheless,  has  some  fundamen- 
tal characteristics  in  common  with  it.  In  Rhabdopleura 
the  individuals  of  a  colony  are  not  independent,  but  are 
connected  with  each  other  by  a  common  cord  or  canlus, 
which  represents  the  remains  of  the  contractile  stalks  of 
the  polyps.  As  the  growth  of  the  colony  proceeds,  the 
distal  portions  of  the  stalks  {i.e.  the  portions  farthest  away 
from  the  animals)  become  shrunken  and  hard.  The  buds 
arise  from  the  soft  portions  of  the  caul  us,  and  never  be- 
come detached  as  they  do  in  the  case  of  Cephalodiscus. 
There  is  only  a  single  pair  of  tentacular  plumes  in  Rhab- 
dopleura. 

FowLKR  has  recently  shown  that  in  Rhabdopleura  the 
coelom,  whose  existence  was  first  established  by  La\- 
kesti:k,  exhibits  the  same  subdivisions  as  have  been 
mentioned  above  for  Cephalodiscus ;  namely,  ( i )  the  cavity 
of  the   large   buccal  shield,  (2)  the   collar-cavity  opening 


PR^EORAI.   LO/iE. 


267 


to  the  exterior  by  a  pair  of  dorsally  placed  collar-pores, 
and  (3)  the  body-cavity  proper  surroundinj^  the  alimentary 
canal.  AccordinL;  to  Fowler,  who  has  recently  ilescrihed 
them  in  Khabdopleura,  the  nervous  system  and  notochord 
have  essentially  similar  relations  to  those  which  obtain  in 
Cephalodiscus,  but  there  are  no  proboscis-pores  ami  no 
gill-slits. 


he 

Ix- 

m 

Itv 


ng 


TIIK   rU/i:()R.\L   LOBK   OF   ECHINODKRM    I.ARV.K. 

In  the  previous  pages  a  good  deal  of  stress  has  been 
laid  on  the  existence  of  a  prxoral  lobe  in  the  various  types 
considered.  We  have  recognised  it  in  the  snout  of  Am- 
phioxus  (prccoral  coelom  +  pr?eoral  pit),  in  the  proboscis 
of  Balanoglossus,  the  fixing  organ  of  the  Ascidian  tai!!)ole, 
and  in  the  buccal  shield  of  Ctphalodiscus  and  Rhabdo- 
pleura. 

F'rom  a  morphological  standpoint  the  pmeoral  lobe  is 
probably  one  of  the  most  important,  as  it  is  certainly  one 
of  the  oldest,  structures  of  the  body  of  bilateral  animals, 
and  it  becomes,  therefore,  a  matter  of  the  first  moment  to 
be  able  to  trace  the  modifications  which  it  has  undergone 
along  the  different  lines  of  evolution  which  have  culmi- 
nated in  the  existing  types  of  animal  life.  The  subject  is 
a  very  large  one,  and  can  only  be  treated  here  in  its 
broadest  outlines. 

It  is  now  very  generally  admitted  by  zoiJlogists  that  the 
Echinoderms  (star-fishes,  sea-urchins,  etc.)  owe  the  radial 
symmetry,  which  is  one  of  the  most  obvious  characteristics 
of  their  organisation,  to  their  having  been  derived  from 
bilaterally  symmetrical  ancestors,  which  became  adajited 
to  a  fixed  or  sessile  existence.  If  this  view  is  ccjrrect, 
and  there  is  good  reason  for  supposing  that  it  is.  it  follows 
that  the  majority  of  living  ICchinoderms  have  secondarily 


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THE   PR  0  TO  CHORDA  TA. 


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lost  their  sessile  mode  of  existence,  and  have  again  become 
froc-living,  retaining,  however,  their  radial  symmetry.  At 
the  present  time  the  fixed  habit  of  life  is  only  retained 
by  the  members  of  one  of  the  subdivisions  of  the  Echino- 
derm  class  ;  namely,  the  Crinoidca. 

Most  genera  of  Crinoids  {R/iiaocrinns,  Pcntacrinus,  etc.) 
remain  fixed  by  a  long,  jointed  stalk  throughout  life  ;  but 
the  well-known  "feather-star,"  Antcdon  rosacea,  is  only 
fixed  during  a  certain  period  of  its  larval  development.  At 
the  close  of  the  period  ot  fixation  the  body  cf  the  animal, 
or,  as  it  is  called,  the  calyx,  breaks  away  from  the  stalk  by 
which  it  was  attached  to  the  rocks,  and  so  begins  to  lead  a 
free  existence,  being  capable  of  swimming  vigorously  by 
the  flapping  of  its  arm.s. 

Although  the  existing  Crinoids  have  become  extensively 
modified  along  their  particular  line  of  evolution,  yet  there 
is  reason  to  believe  that  they  represent  the  more  im- 
mediate descendiints  of  the  primaeval  form  which  ex- 
changed its  primitively  free  life  and  bilateral  symmetry  for 
a  sessile  existence  and  radial  symmetry.  This  view  is 
strengthened  by  the  character  of  the  free-swimming  larva 
of  Antedon.  This  larva  does  not  possess,  in  any  extrava- 
gant degree,  those  fantastic  structures  which  are  so 
characteristic  of  other  Echinoderm  larvre,  such  as  the 
provisional  ciliated  processes  or  arms  of  the  "  Pluteus " 
(larva  of  sea-urchins),  or  the  undulating  ciliated  bands  of 
Auricularia. 

On  the  contrary,  the  larva  of  Antedon  is  a  simple 
barrel-shaped  organism,  with  regular  ciliated  bands  pass- 
ing around  it  (Fig.  128). 

Perhaps  the  structure  which,  above  all,  stamps  the  free- 
swimming  larva  of  Antedon  as  having,  from  a  phylogenetic 
point  of  view,  a  more  primitive  type  of  organisation  than 


PRj^  ^KAL   LOBE. 


260 


that  of  Other  Echinoderm  larvc-e,  is  the  well-developed 
apical  plate  at  its  anterior  extremity.  We  may  express 
this  in  other  words  by  saying  that  the  larva  of  Antedon 
possesses  a  central  nervous  system  at  the  apex  of  its 
praeoral  lobe.  That  the  pra^- 
oral  lobe  in  this  larva  is  not 
sharply  marked  off  from  the 
rest  of  the  body  is  a  detail 
of  no  morphological  signifi- 
cance. 

The  apical  nervous  sys- 
tem of  the  Antedon  larva 
was  discovered  in   1888  by 

H.      Bury,     and     has     been         Fig.   laS.-FrLc-swimming  larva  of 

more     clearly     brOUo-ht     out    '^'''^'^'"'  *'"^'^(^'i<  from    the  ventral   side. 
;'  *=•  (After  Skeli(;ek.) 

and  emphasised  in  a  recent       "A  Apical  pole.  c.b.  Ciliated  bands. 

«r<^r^  Kw  f^.-   r»c-,.. .  V  ^  c  -^^  ^'^'"g  tJisc.    V.  Vestibulum  (so-called 

work  by  Dr.  Oswald  SeKIJ-    larval    mouth,    althoush    at    this    stage 

GER.      At  the  point  which  is    "^""^'^  ^^  ectodermic  groove). 

marked  externally  by  the  anterior  tuft  of  long  cilia  in 
Fig.  129  there  is  a  slight  groove  in  the  ectoderm  below 
which  nerve-fibres  and  ganglion-cells  can  be  identified. 
Seeliger  further  describes  a  pair  of  longitudinal  nerves 
running  from  the  nervous  area  of  the  apex  along  the 
ventro-lateral  margins  of  the  body. 

As  already  indicated,  the  apical  plate  is,  as  a  general 
rule,  conspicuous  by  its  absence  in  the  typical  Echinoderm 
larva.  In  the  free-swimming  larva  of  Antedon,  however, 
it  is  emphatically  present,  although  destined  to  become 
entirely  aborted  after  the  fixation  of  the  larva. 

In  most  Invertebrate  larvoe  in  which  an  apical  plate  is 
present  {e.g.  the  Trochophore-larva  of  Annelids  and  Mol- 
luscs) it  becomes,  during  the  metamorphosis,  involved  in 
other  ectodermic  thickenings  of  the  praeoral  lobe,  which 


i  \ 


2^o 


THE  PR  0 1 V  CHORDA  TA. 


M^^l- 


w 


collectively  give  rise  to  the  cerebral  or  supraoesophageal 
ganglion.  The  apical  plate  may  thus  be  defined  as  a 
primitive  central  nervous  system  at  the  apex  of  the 
praeoral  lobe,  being  the  forerunner  and  formative  centre 
of  the  cerebral  ganglion  oi  the  Invertebrates. 

Although,  with  the  exception  of  the  Crinoids,  there  is 
no  apical  plate  in  the  typical  Echinoderm  larva,  yet,  as 
noted  above,  in  many  cases  a  curious  transitory  lengthen- 
ing of  the  ectodermic  cells  at  the  apical  pole  has  been, 

and  can  be  without  great 
difficulty,  observed  in  larvae 
of  star-fishes  and  sea-urchins. 
This  alone  would  seem  to 
indicate  the  forme-  exi.st- 
ence  of  a  central  nervous 
system  at  the  apex  of  the 
pracoral  lobe  in  the  bilateral 
ancestor  of  the  Echinoderms. 
The  way  in  which  the 
Fig.  139.— Larva  of. 4.f/<rr/»fl^/*^r7j<i,  primary   blastoccclic    cavity 

viewed  as  a  transparent  object  from  the 

left  side.    (After  Luinvic.)  of  the  pricoral  lobe  can  be 

.;,/...  Enteric  cavity     /^.  Left  entcro.  replaced    by    a  dilatation    of 
CGL'I,  communicating  with  the  right  entero-         ^                   J 

coL'l  through  p.l,  the  praeoral   lobe.    st.  the   CnterOCOel  haS    bccn  dC- 

Stomodoeum.  •,11  «       1     r       <t> 

scribed  above,  both  for  Tor- 
naria  and  for  the  larva  of  Astcrias  vN/i^aris  (Figs.  » 21-122), 
In  some  cases,  as  in  Astcriua  gibbosa,  the  prrcoral  lobe  is 
occupied  by  the  enteroccel  from  the  very  beginning.  In  the 
"Pluteus"  larva  of  the  Echinids  (sea-urchin.s)  the  praeoral 
lobe  is  much  reduced  ;  but  in  other  Echinoderms,  as  in 
the  singular  larva  of  Asterina  i^ibbosa,  and  in  the  so-called 
Brachiolaria-larva  of  the  Asterids  (star-fishes)  in  general,  it 
is  very  prominent,  and  serves  as  an  effective  locomoton 
{creeping)  organ. 


PA'  EORAL   LOHE. 


271 


The  very  interesting  observation  has  recently  been 
made  by  MacBride,  that  the  larva  of  Astcrina  gibbosa 
actually  undergoes  temporary  fixation  at  the  beginning  of 
the  metamorphosis,  the  fixation  being  effected  by  the 
prceoral  lobe  in  a  manner  strikingly  similar  to  that  of  the 
larva^  of  Antcdon  and  of  Ciona. 

In  the  larva  of  Antedon  the  adhering  disc,  by  which  the 
larva  eventually  fixes  itself  to  some  foreign  surface,  is 
placed  near  the  front  end  of 
the  prcxoral  lobe  immediately 
below  the  apical  plate. 

The  central  nervous  sys- 
tem of  the  adult  Echinoderm 
arises  in  entire  indepen- 
dence of  the  actual  or  sup- 
pressed apical  nervous  sys- 

tem  Ot   the  larva,  and   not  at    viewed  as  an  opaque  ..bject  fro...  tl»e  left 

all  from  the  ectoderm  of  the  '''*^-    *-^^""''  li'dwic) 

pr.l.  Traioral  lobe. 

prneoral  lobe. 

We  have  thus  seen  how  within  the  limits  of  a  single 
group  (viz.  the  Echinoderms)  the  pra^oral  lobe  can  become 
completely  emancipated  from  the  central  nervous  system  ; 
and  we  have  further  recognised  the  fact  that  whether  the 
cavity  of  the  prai^oral  lobe  is  a  derivative  of  the  primary  or 
secondary  body-cavity,  whether  it  contains  loose  mesen- 
chyme or  is  lined  by  an  endothelium,  the  morphological 
value  of  the  praeoral  lobe  itself  remains  the  same. 

THE   rR.T.ORAL    LORE  OF  THE   PROTOCIIORDATES. 

It  is  probable  that  the  misunderstandings  and  disagree- 
ments which  are  of  such  frequent  occurrence  among  mor- 
phologists  with  regard  to  the  comparison  of  the  types  of 
central    nervous   system    presented    respectively    by    the 


Ih. 


[f   ' 


3'  i 


272 


T//£  PKOTOCIIOKDATA. 


i!k  ■ 


Vertebrates  and  the  Invertebrates,  are  largely  due  to  the 
failure  to  detect  some  general  principle  of  evolution  to 
which  that  archaic  structure,  the  proeoral  lobe,  has  been 
subjected. 

Nevertheless,  there  are  many  indications  which  point 
irresistibly  to  the  conclusion,  which  I  have  recently 
brought  forward,  that  the  prime  factor  which  must  be 
recognised  in  the  evolution  of  the  prasoral  lobe,  from  the 
relations  which  it  presents  in  the  Invertebrates  to  those 
which  it  holds  in  the  Protochordates  and  Vertebrates,  is 
its  emancipation  from  the  central  nen>ous  system. 

In  the  great  groups  of  the  Annelids,  Molluscs,  and 
Arthropods,  the  praeoral  lobe  (prostomium,  procephalic 
lobe)  is  essentially  the  seat  of  the  brain  or  cerebral  gan- 
glion. The  latter,  through  its  representative,  the  apical 
plate,  is  the  main  and  often  the  sole  element  of  the  central 
nervous  system  in  the  Trochophore-larva  of  Annelids  and 
Molluscs.* 

*  In  speaking  of  the  apical  plate  as  the  forerunner  or  formative  centre  of 
the  cerebral  ganglion,  it  must  not  he  assumed  that  these  are  not  distinct 
structures.  The  apical  plr.te  is  essentially  median  and  unpaired,  while  the 
cerebral  ganglion  is  paired.  They  can  both,  however,  be  included  under  the 
general  term,  apical  nervous  system,  since  they  arise  from  the  ectoderm  of 
the  prceoral  lobe.  On  the  other  hand,  the  cerebral  ganglion  may  arise  inde- 
pendently of  an  apical  plate;  as,  for  instance,  in  Ltimhricus,  where  there  is 
no  apical  plate,  or  in  the  Nemertines,  where  the  apical  plate  is  discarded 
together  with  other  larval  structures  (Pilidium).  Again,  as  in  Lumbricus  and 
many  other  cases,  the  cerebral  ganglion,  after  having  separated  from  the 
ectoderm  of  the  pra;oral  lobe,  may  recede  backwards  for  a  considerable  dis- 
tance, so  as  not  to  lie  in  the  prreoral  lolie  in  the  adult.  It  is  possible  that  the 
position  of  the  cerebral  ganglia  of  Nemertines  may  be  accounted  for  by  some 
such  phylogenetic  recession  from  the  pncoral  lobe. 

If  necessary,  it  might  be  saitl  that  the  pra'oral  lobe  can  acquire  emancipa- 
tion from  the  central  nervous  system  by  a  simple  recession  of  the  cei'ebral 
ganglion.  In  the  case  of  the  Protochordates,  however,  on  the  view  here  advo- 
cated, ihe  prceoral  lobe  has  acquired  emancipation  from  the  central  nervous 
system,  not  by  the  mere  recession,  but  by  the  complete  disappearance  of  the 
Inve/tebrate  cerebral  ganglion. 


PR.^.ORAL  LOBE. 


■/  5 


dis- 
the 


jme 


lipa- 
Ibral 
llvo- 

i'OUS 

the 


At  a  later  stage  of  development  the  longitudinal  nerve- 
cord  (confining  the  description  to  the  Annelids  for  the 
sake  of  simplicity)  arises  independently  of  the  cerebral 
ganglion,  from  a  pair  of  longitudinal  thickenings  of  the 
ectoderm  near  the  mid-ventral  line,  becoming  secondarily 
connected  with  the  cerebral  ganglion  by  the  circumoesoph- 
ageal  nerve-collar  or  commissure. 

As  already  indicated,  it  seems  probable,  as  was  sug- 
gested by  Balfour  and  Gegenuaur,  that  the  ventral 
nerve-cord  of  the  Annelids  is  to  be  regarded  as  having 
arisen  phylogenetically  by  the  mutual  approximation  of 
two  such  lateral  cords  as  occur  in  the  Nemertines,  and 
like  the  latter  may  be  supposed  to  have  originated  by  a 
concentration  on  the  ventral  side  of  the  body  of  that 
primitively  continuous  sub-epidermic  nerve-plexus  which 
is  such  a  characteristic  feature  of  the  Nemertines.  From 
a  consideration  of  the  adult  nervous  system  in  the 
Echinoderms,  Nemertines,  Enteropneusta  (Balanoglossus), 
Annelids,  and  Molluscs,  it  is  evident  that  such  a  con- 
centration of  nervous  tissue  has  from  first  to  last  occurred 
along  very  different  lines. 

Speaking  in  broad  terms,  it  may  be  said  that  the  only 
portion  of  the  Invertebrate  nervous  system  which,  in  its 
prime  essence,  is  invariable  and  universal  (due  allowance 
being  made  for  exceptional  cases)  is  the  cerebral  ganglion 
or  its  forerunner,  the  apical  plate,  the  seat  of  which  lies  in 
the  pra^oral  lobe.^ 

Under  these  circumstances  it  will  suffice  to  confine  our 
attention  to  the  prneoral  lobe,  in  the  belief  that  if  an 
understanding  can  be  arrived  at  with  regard  to  that  impor- 
tant structure,  one  of  the  chief  difficulties  in  the  way  of  a 
just  conception  of  the  relations  existing  between  Verte- 
brates and  Invertebrates  will  have  been  overcome. 


^ 


-74 


THE   PRO  TOCIIORDA  TA. 


%l 


Returning  now  to  Balanoglossus,  we  have  to  remark 
that  in  the  Tornaria  larva  the  central  nervous  system  is 
represented  entirely  by  the  apical  plate  of  the  praioral 
lobe,  the  situation  of  the  apical  plate  corresponding  to  the 
anterior  tip  of  the  proboscis  of  the  adult.  Unlike  the 
Annelids,  however,  the  apical  plate  of  Tornaria  does  not 
become  replaced  after  the  manner  of  the  Invertebrates  by 
the  development  of  a  cerebral  ganglion  arising  like  it  from 
the  ectoderm  of  the  pr^eoral  lobe  and  with  it  as  a  formative 
centre.  On  the  contn.ry,  it  completely  disappears  after 
the  metamorphosis,  having  become  replaced  physiologically 
by  the  development  of  the  medullary  tube  in  true  Verte- 
brate fashion  from  the  dorsal  ectoderm  of  the  collar-region 
behind  the  prjeoral  lobe.* 

In  the  Ascidian  larva,  however,  and  in  Amphioxus,  the 
characteristic  Invertebrate  apical  nervous  system  no  longer 
appears  in  any  stage  of  development,  its  physiological  func- 
tion having  been  once  for  all  assumed  by  the  medullary 
tube  (cerebral  vesicle  +  spinal  cord)  which  lies  par  excel- 
lence behind  the  praeoral  lobe  (Fig,  131). 


Anterior  and  Posterior  Neurenteric  Canals,  and  the 
Position  of  the  Mouth  in  the  Protochordatcs. 

After  the  postoral  medullary  tube  had  led  indirectly  to 
the  complete  obliteration  of  the  praeoral  apical  nervous 
system,  and  had  attained  to  such  a  degree  of  development 
as  we  find,  for  instance,  in  the  Ascidian  tadpole,  the  central 
canal  of  the  cerebro-spinal  nervous  system  appears  to 
have  acquired  remarkable  relations  with  the  alimentary 
canal.     At  both  ends  of  the  body  connecting  ducts   be- 


*  For  a  detailed  account  of  the  formation  of  the  medullary  tube  in  the  col- 
lar-region of  Balanoglossus  see  Morgan  (Bibliography,  Nos.  124  and  125). 


I'K.EOKAL  LOHK. 


^75 


ca,ne  established  between  the  nervous  an<l  digestive 
systems,  known  respectively  as  the  anterior  and  fostcricr 
'leiircuUrK  canals. 

The   posterior   neurenteric  canal   is  only  of  transitory 
occurrence  m  all  existing  Vertebrates,  and  leads  from  the 


J  p.1     enJ  ^  ftp   nc 


(After  WILLEY.)  ^    ^  "'  Amph.oxus,  and    (C)    Balanogiossus. 

chord      ..  Eye/./    Oto^ys.     W    ''^//^P^'-'^- ,  "V^'""^""^^-^ '"^e.    .^.  Noto- 
Balanoglossus  ^       ^ '  ^""^  ''•  P'-^^oscis-gland  and  proboscis-heart  of 


Jlk 


276 


THE  rROTOCJIORDA  J. I. 


-J^c 


x,d 


Fig.  132.  —  Sagitta  hexaptera  from  the 
ventral  su'-^ace ;  nearly  three  times  natural 
.size.     (After  O.  Hkrtwic.) 

a.  Anus.  bc'^.  Head-cavities.  bc"^. 
Trunk-coL'lom.  bc^.  Caudal  coelom.  c.L 
Caudal  septum,  com.  Commissure,  from 
the  cerebral  ganglion  to  the  single  ventral 
ganglion.  /',/'-,/''•  Fins.  m.  Mouth. 
(>.(/.  Oviduct,  ov.  Ovary,  sp.  Prehen- 
sile bristles,  s.v.  Seminal  vesicle.  /.  Tes- 
tis,   v.g.  Ventral  ganglion. 


neural  tube  into  the  extreme 
posterior  end  of  the  aliment- 
ary canal ;  in  fact,  into  that 
portion  of  it  which,  in  the 
embryos  of  the  higher  forms, 
iS  known  as  the  post-anal 
gut.  The  anterior  neuren- 
teric  canal,  in  its  most  primi- 
tive  condition,  opens  into  the 
base    of    the    buccal    tube 

(Fig.  131)- 

On  this  account  we  find 
in  the  Ascidian  tadpole  that 
the  mouth  is  no  longer  ven- 
tral, as  it  is  in  Balanoglossus, 
but  is  placed  dorsally,  im- 
mediately in  front  of  the 
anterior  extremity  of  the 
medullary  tube.  This  in- 
timate relation  between  the 
mouth  and  the  central  ner- 
vous system  gives  a  reason 
for  the  contrast  between  the 
dorsal  position  of  the  mouth 
in  the  Ascidian  tadpole  and 
its  ventral  position  in  Bala- 
noglossus. 

In  Amphioxus  we  have 
seen  that  the  mouth  has  been 
forced  aside  from  its  more 
primitive  dorsal  position  by 
the  forward  extension  of  the 
notochord  to  the  tip  of  the 


rh'.EOKA/.  I.  OF  I-:, 


277 


piiL'oral  lobe.  The  origin  of  the  main  cavity  of  the  pnt- 
oral  lobe  in  Amphioxus  from  the  right  of  a  symmetrical 
pair  of  head-cavities  (anterior  intestinal  diverticula  of 
Hatschek)  has  becii  described  in  a  previous  chapter.  In 
Halanoglossus  there  is  no  such  complete  division  of  the 
prx'oral  body-cavity,  but  it  is  throughout  a  single  space, 
its  right  and  left  halves  being  confluent.  If  we  now  com- 
pare the  condition  of  things  in  the  embryo  of  Amphioxus. 
where  we  have  a  symmetrical  pair  of  head-cavities,  with 
that  of  some  other  form  which,  in  the  adult  condition, 
possesses  a  distinct  pair  of  such  cavities,  it  may  assist  us 
in  imagining  how  the  mouth  could  have  assumed  such 
opposite  relations  as  have  been  mentioned  above. 

But  first  it  may  be  pointed  out  that  in  Appctidiculayia, 
where,  as  it  would  appear,  in  correlation  with  the  second- 
ary acquirement  of  a  purely  pelagic  habit  of  life  (although 
this  point  of  view  is  not  shared  by  such  authorities  as 
Herdman,  Seeliger,  and  Brooks),  the  pracoral  lobe  has 
been  reduced  to  a  minimum,  or  to  zero,  the  mouth  has 
thereby  come  to  lie  in  a  terminal,  or  sub-terminal,  position, 
with  a  slight  tendency  towards  the  dorsal  side.* 

In  the  curious  pelagic  worm,  Sagitta,  we  meet  with 
another  instance  of  an  animal  in  which  the  prneoral  lobe, 
in  the  ordinary  sense  of  the  term,  is  reduced  to  a  mini- 
mum, and  the  mouth  has  therefore  a  sub-terminal  position, 
with  a  ventral  inclination  (Fig.  132).  But  although  there 
is  no  distinct  prreoral  lobe  in  Sagitta,  there  is,  neverthe- 
less, a /rt;/;'t9/"/rrrt'</-r^?77'//Vi',  which  are  directly  comparable, 
if   not   perfectly   homologous,  with    the   above-mentioned 

*  Whatever  ihe  truth  may  be  as  to  the  precise  systematic  position  and 
phylogenetic  value  of  Ajiiiendicularia,  one  thinj^,  to  my  mind,  remains  aliso- 
lutely  certain,  namely,  that  it  has  descended  from  a  ft)rm  which  possessed  a 
prixioral  lobe,  and  that  it  has  secondarily  lost  tliat  structure. 


F  'I 


J 1 


r- 


278 


Till:    rROTOCnORDA  lA. 


head-cavities  of  Amphioxus,  although  they  have  a  some- 
what different  origin. 

It  should  not  be  forgotten  that  Sagitta  occupies  a  very 
isolated  position  in  the  zoological  system,  being  placed  in 
a  group  by  itself,  the  ChcEtognatha,  and  that  therefore  the 
peculiarities  of  its  organisation  cannot  be  taken  as  repre- 
senting any  definite  intermediate  stage  in  the  phylogeny 
of  other  forms,  yet,  from  a  general  standpoint,  the  con- 
ditions which  it  presents  in  its  life-history  are  highly 
instructive. 

The  head-cavities  of  Sagitta  arise  by  constriction  from 
the  anterior  extremities  of  the  single  pair  of  archentcric 
pouches  which  give  rise  to  the  coelom  of  the  adult.  They 
remain  distinct  and  separate  on  either  side  of  the  head 
throughout  life.  If,  now,  we  imagine  them  to  grow  for- 
ward and  fuse  together  in  front  of  the  mouth,  in  a  simi- 
lar manner  to  that  described  above  for  the  enteroccelic 
pouches  of  Asterias,  we  should  have  a  prxjoral  body-cavity 
of  a  similar  character  to  that  of  Balanoglossus. 

Now,  the  ultimate  position  of  the  mouth  under  these 
new  conditions  would  depend  upon  circumstances  affect- 
ing the  whole  organisation  of  the  animal. 

In  an  animal  whose  grade  of  organisation  was  on  an 
approximate  level  with  that  of  Sagitta  the  mouth  would 
undoubtedly  remain  on  the  ventral  side  of  the  body.  But 
in  an  animal  whose  organisation  had  reached  the  stage 
of  evolution  represented  by  that  unknown  ancestor  of 
Amphioxus  (most  nearly  represented  at  the  present  time 
by  the  Ascidian  tadpole),  whose  notochord  did  not  extend 
beyond  the  anterior  limit  of  the  neural  tube,  the  mouth 
would  pass  to  the  dorsal  side  of  the  body  to  come  into 
connexion  with  the  neural  canal. 


PK.I'lOK.ir.   I.OliE. 


279 


THK   PR.EORAL   LOBE   IN   THE  CRANIATE  VERTEIJRATLS. 

After  what  has  been  said  above,  in  this  and  the  preced- 
ing chapters,  the  question  as  to  how  the  prxv.  .'  lobe  's 
represented  in  the  craniate  Vertebrates  need  not  detain  us 
long. 

Since,  as  shown  above,  the  nervous  element  of  the  prrc- 
oral  lobe  (apical  i)late  and  cerebral  ganglion)  is  entirely 
lacking  in  the  Vertebrates,  we  can  only  expect  to  find  the 
mesodermal  element  represented  in  the  head-cavities  of 
the  higher  forms. 

In  consequence  of  the  great  development  of  the  brain, 
even  in  the  lowest  craniate  Vertebrates,  as  compared  with 
Amphioxus,  and  in  consequence  too  of  the  cranial  flexure, 
the  head-cavities  have  been  made  to  assume  a  more  sub- 
ordinate position,  and  no  longer  take  part  in  the  formation 
of  a  prominent  lobe  in  front  of  the  body.  This  is  a  perfect 
illustration  of  "  le  principe  du  balancement  des  organes  " 
of  Geoffroy  Saint-Hilaire,  the  prncoral  lobe  decreasing  as 
the  brain  increases.  A  comparison  between  Figs.  70, 
72,  117,  and  135  will  show  at  once  that  the  pr?coral  head- 
cavities  of  Amphioxus  and  Halanoglossus  arc  the  homo- 
logues  of  the  pnemandibular  head-cavities  of  the  craniate 
Vertebrates. 

These  cavities  lie  at  first  below  the  mid-brain,  and  later 
their  walls  give  rise  to  most  of  the  eye-muscles.  In  Figs. 
91  and  135  the  median  portion  of  the  praimandibular 
cavities  can  be  seen  still  in  the  form  of  an  anterior  pocket 
of  the  endoderm,  and  it  may  be  noticed  how  far  it  is 
removed  from  the  anterior  extremity  of  the  body  to  which 
it  extends  in  Amphioxus,  etc.  In  the  craniate  Verte- 
brates the  brain  extends  forwards,  and  the  head-cavities 


I'.t ' 


';,    s 


280 


77/iS  PROTOCHORDATA. 


remain  behind.  This  is,  as  we  should  expect,  the  exact 
reverse  to  what  obtains  in  Amphioxus. 

In  connexion  with  the  evolution  of  the  prseoral  lobe, 
we  thus  have  an  excellent  example  of  repeated  change  of 
function. 

We  may  conclude,  therefore,  that  the  prneoral  lobe, 
which,  in  the  Invertebrates,  is  above  all  the  bearer  of  the 
cerebral  ganglion,  and  in  the  Protochoi  dates  is  released 
from  this  function  and  becomes  in  part  a  locorr,otor 
(Balanoglossus,  Ccphalodiscus)  fixing  (Ascidian)  and  bur- 
rowing (Amphioxus)  organ,  is  represented  in  the  craniate 
Vertebrates  by  the  prccniandibiilar  head-cavities,  whose 
walls  give  rise  to  most  of  the  eye-muscles. 


M-'o    "'si 


'■  $i 


% 


thp:  mouth  of  the  cr.\ni.\te  vertebr.vtes. 

In  consequence  of  the  increase  in  the  size  of  the  brain, 
its  forward  extension  and  its  cranial  flexure,  together  with 
the  relative  reduction  of  the  head-cavities,  it  is  obvious 
that  the  mouth  has  been  carried  round  from  its  pHmitively 
dorsal  position  to  its  final  position  on  the  ventral  side  of 
the  head  in  the  craniate  Vertebrates.  (Cf.  Fig.  91.)  This 
would  have  been  all  that  need  be  said  about  the  mouth 
were  it  not  for  the  fact  that  the  view,  originally  started  by 
DoMRN,  that  the  Vertebrate  mouth  was  a  new  formation 
resul*^ing  from  the  fusion  of  two  gill-slits,  has  received  such 
wide  support  and  still  in  a  measure  holds  its  own. 

Since  the  Annelid  mouth  perforates  the  central  nervous 
system  in  passing  through  the  circumoesophageal  nerve- 
collar,  it  was  necessary  to  frame  a  theory  which  would 
get  over  the  dilficulty  that  nothing  of  the  kind  occurs  in 
the  Vertebrates.  Accordingly  Dohrn  supposed  that  the 
old  Annelid  mouth  had  become  aborted,  and  was  replaced 


MOUTH. 


281 


by  a  new  mouth  derived  from  a  fusion  across  the  mid- 
ventral  line  of  a  pair  of  gill-clefts.  Dohrn  was  a  trifle 
uncertain  as  to  the  rudiment  of  the  old  mouth,  but  Bevkd 
was  more  certain  on  this  point,  and  thought  he  had  estab- 
lished the  fact  that  the  hy- 
pophysis cerebri  represented 
the  remains  of  the  old  An- 
nelid mouth. 

Dohrn  certainly  succeeded 

in    bringing   forward    some 

apparently  good  evidence  in 

support  of  his  theory  of  the 

gill-slit  origin  of  the  mouth. 

This  evidence  was  derived 

from   the  study  of   the  de- 
velopment of  the  mouth  in 

Teleostean  or  bony  fishes  «• 

T  _    ,  *ig-  133-  —  Two  frontal  views  of  an 

m  many  leleosteans  the  embryo  of  Hatmc/tus  fa,,,  to  show  tin- 
mouth  ha<s  nf  firct-  ^r,  ^  ^'ouWe  nature  of  the  stomodoeum.  (From 
mOUtn  nas  at  hrst  an    appar-    hitherto  unpublished  drawings  kindly  lent 

ently  double  origin,  in  that  ^^  ^''''  ^-  ^'-  ^'""'''-^ 

I  he    -mbrvo  is  Ivinp  upon  the  volk 

two  separate  ectodermal  in-  '^^'^  '^e  septum  which  divides  the  stomo- 

growths  occur  which  fuse  .tre'':r;L\™,irj,iit r;: 

with  the  endoderm,  instead   '^/  ^^^^-  '^''•^  '°'^'=''  fig'"'e '« a  drawing 

^r      .,  ,.  °'  ^^^  same  embrvo  as  the  upper  a  few 

Ot      the     median      Stomodoeal     hours  later.     AhoC-e  the  stomodreum  are 

involution  which  is  so  char-  tT'"  "."'"  "?"''"  "''''''''  ''"'  (•■"aliments  of 

WHICH  i.>    bO  cnar-    the  external  nares),  and  at  the  sides  of 
acteriStic     of     other     Verte-    *'^^  ^^^'^  ^""^  '^'^  rudiments  of  the  eyes. 

brates.  This  double  origin  of  the  mouth  is  particularly 
well  shown  in  the  embryos  of  the  remarkable  toad-fish, 
Batrachus  tan,  as  observed  by  Miss  Cornelia  Clapp  at 
the  Marine  Biological  Laboratory  of  Woods  Holl,  Mass., 
in  1889  (Fig.  133)-  In  this  case  the  mouth-cavity  is  seen 
to  be  divided  into  two  halves  by  a  median  septum. 

Subsequently  the  septum   becomes  absorbed,   and   the 


282 


THE  PROTOCIIORDATA. 


i;  : 


r'.'""  '+ 


two  halves  of  the  mouth  coalesce.  In  view  of  the  pre- 
vious existence  of  the  gill-slit  theory  of  the  mouth,  some 
such  theory  being  a  necessary  accessory  to  the  Annelid- 
theory,  it  is  not  surprising  that  this  undoubted  double 
origin  of  the  mouth  in  Teleosteans  should  be  regarded  as 
a  striking  confirmation  of  Dohrn's  hypothesis.  An^  yet, 
occurring  as  it  does  only  in  the  Teleosteans,  whose  devel- 
opment is  admittedly  in  many  respects  highly  modified, 
the  interpretation  which  Dohrn  and  his  followers  have 
placed  upon  this  observation  must  always  have  been  open 
to  c'oubt.  The  simplest  explanation  of  the  double  origin 
of  the  Teleostean  mouth  is  that,  owing  to  certain  condi- 
tions (possibly  mechanical)  of  development,  the  two  angles 
of  the  mouth  develop  before  the  median  portion.  This  is 
the  conclusion  which  H.  B.  Pollard  has  also  reached  in 
his  recent  studies  on  the  development  of  the  head  in  the 
Teleostean  fish,  Gobius  capita. 

According  to  the  standpoint  I  have  adopted  in  the  fore- 
going pages,  there  is  no  a  priori  reason  for  doub:ing  that 
the  Vertebrate  mouth  is  completely  homologous  with  the 
Protochordate  mouth ;  and  that  the  latter  in  its  turn  is 
the  direct  descendant  of  the  typical  Invertebrate  mouth. 

Again,  the  anatomy  and  development  of  the  Protochor- 
dates  and  of  tne  Cyclostomi  (Ammoccetes)  show  no  indica- 
tion whatever  of  a  discontinuity  in  the  evolution  of  the 
most  highly  elaborated  mouth  of  the  gnathostomous  or 
jawed  Vertebrates. 

We  conclude,  therefore,  that  the  ventral  mouth  of  the 
craniate  Vertebrates  is  the  homologue  of  the  primordial 
dorsal  mouth  as  we  find  it  in  the  Protochordates,  and  that 
its  direction  of  evolution  has  been,  as  was  so  ably  main- 
tained by  Balfour,  from  the  cyclostomous  to  the  gnatho- 
stomous condition. 


HYPOPHYSIS. 


283 


SIGNIFICANCE   OF  THE   HYPOPHYSIS  CEREBRI. 

The  pituitary  body,  or  hypophysis,  belongs  to  the  series 
of  ductless  "glands"  (pineal  body,  thyroid  gland,  thy- 
mus, etc.)  which  are  such  a  characteristic  feature  of  the 
vertebrate  organisation.  It  arises  as  an  ectodermal  invo- 
lution from  the  roof  of  the  stomodoeum,  directed  towards 
the  base  of  the  primary  fore-brain,  from  which  the  infun- 
dibulum  grows  out. 

The  pituitary  involution  becomes  in  most  forms  nipped 
off  from  the  stomodoeum,  and  then  lies  as  a  closed  sac 
in  contiguity  with  the  infundibulum.  Later  on  it  produces 
a  system  of  branches,  the  lumina  of  which  tend  to  dis- 
appear ;  and  in  some  forms  {e.g.  Mammalia)  it  undergoes 
actual  fusion  with  the  infundibulum. 

The  very  constant  relation  of  the  hypophysis  to  the 
infundibulum  in  the  craniate  Vertebrates  (see  Fig.  134) 
naturally  led  to  the  supposition  that  there  must  orrginally 
have  been  a  functional  connexion  between  the  two  struct- 
ures of  a  similar  nature  to  that  which  exists  between  the 
olfactory  pit  and  neuropore  in  Amphioxus.  Recent  re- 
searches, however,  have  rendered  it  probable  that  such  a 
supposition  is  erroneous.  Von  Kupffer  has  discovered 
the  homolocrue  of  the  lobus  olfactorius  of  Amphioxus  in 
the  craniate  Vertebrates,  and  has  shown  that  it  occurs  at 
a  point  far  removed  from  the  infundibular  region. 

Until  recently  it  was  also  very  generally  thoughc  that 
the  infundibulum  represented  the  anterior  end  of  the 
brain,  which  had  become  bent  downwards  and  backwards 
by  the  cranial  flexure.  Kupffer,  however,  has  brought  for- 
ward weighty  reasons  for  doubting  this  view.  According 
to  him,  the  infundibulum  is  essentially  a  downgrowth  or 


r  Ml, 


284 


THE  PKOrOCHOKDATA. 


evagination  from  the  floor  of  the  brair,  occurring  behind 
the  anterior  terminal  extremity  of  the  l^rain. 

It  follows  that  the  morphological  anterior  extremity  of 
the  craniate  brain  coincides  with  the  median  lobns  olfac- 
toriiis  hnpar,  which  also  represents  the  point  of  last  con- 
nexion of  the  medullary  tube  with  the  superjacent  ecto- 
derm. The  lobus  olfactorius  impar  lies  in  the  anterior 
vertical  wall,  which  forms  the  boundary  of  the  primary 
fore-brain  in  front,  known  as  the  lamina  tcrminalis.  Rabl- 
RiJCKHARD  has  also  observed  the  median  olfactory  lobe  in 


•■s 


i 


Fig.  134.  —  Sagittal  section  through  the  head  of  an  embryo  of  Acanthias. 
(After  Rahl-Ruckhard.) 

ax.  Position  of  anterior  commissure,  a'.  Alimentary  canal,  cer.  Cerebellum. 
ch.  Notochord;  the  black  shading  below  the  notochord  indicates  the  aorta. 
f.b.  Fore-brain,  h.b.  Hind-brain,  hy.  Hypophysis,  already  shut  off  from  the 
stomodu-'um  and  lying  as  a  closed  sac  at  the  base  of  inf,  the  infundibulum. 
i.o.  Lobus  olfactorius.  m.  Mouth,  tn.b.  Mid-brain,  o.c.  Optic  chiasma.  p.b. 
Pineal  body  (epiphysis). 

the  Selachian  embryo  (Fig.   134),  and  it  has  since  been 
found  by  Burckhardt  in  other  forms. 

It  can  thus  hardly  be  doubted  that  the  median  rudi- 
mentary olfactory  lobe  of  the  embryos  of  the  higher 
Vertebrates  is  homologous  with  the  lobus  olfactorius  of 
Amphioxus  (F.g.  51),  and,  like  the  latter,  represents  the 
remains  of  the  neuropcre.     In  Amphioxus,  hovever,  the 


HYPOPHYSIS. 


285 


olfactory  lobe  abuts  against  the  olfactory  pit,  and.  in  fact 
in  young  individuals  opens  into  it  by  the  r.europore 
(Fig-  45). 

On    the    view  which   I   have    urged   above,   triat   the 
olfactory    pit    of    Amphioxus    is    homologous   with    the 
hypophysis  cerebri  of  the  craniate  Vertebrates,  it  must 
be  assumed  that  in  the  latter  forms,  the  neuropore  hav- 
ing ceased  to  be  in  any  way  a  functional  organ,  the  hy- 
pophysis,  which  has  likewise  become  (morphologically)  a 
vestigial  structure,  has  been  mechanically  separated  from 
the  neuropore.  with  which  it  was  primitively  in  functional 
connexion.     It  must  be  supposed  that  this  separation  of 
the  hypophysis  from  the  neuropore  has  been  effected  by 
the  more  rapid  downward  growth  of  the  ectoderm  (from 
which  the  hypophysis  arises)  than  of  the  wall  of  the  brain, 
so  that  the  hypophysis  has  been*  carried  farther  round  to 
the  lower  side  of  the  head  than  the  neuropore  (Fig.  135). 
The  re.>son  for  this  unequal  growth  of  the  externa"!  body- 
wall  and  of  the  cerebral  wall  may.  perhaps,  be  sought  for 
in  the  great  and  independent  increase  in  the  cubical  con- 
tents of  the  brain.3 

We  thus  arrive  at  the  conclusion  that  the  present 
relation  of  the  hypophysis  to  the  infundibulum  in  the 
craniates,  however  intimate  it  may  be  in  some  cases,  is, 
nevertheless,  incidental  and  secondary. 

That  this  conclusir  n  is  not  so  strained  as  might  appear 
at  first  sight  is  clearly  shown  by  the  fact  that  the  in- 
fundibulum is  not  the  only  structure  with  which  the 
hypophysis  enters  into  close  relations. 

In  the  exceptional  cases  of  Myxine  and  Bdellostoma, 
for  instance,  the  distal  end  of  the  hypophysis  has  nothing 
to  do  with  the  infundibulum,  but  actually  opens  into  the 
pharynx.     In    these   hag-fishes,    as   also    in    the   lamprey 


286 


THE  PROrOCIIOKDA TA. 


(where  there  is  no  internal  opening  of  the  hypophysis 
into  the  pharynx),  the  external  opening  of  the  hypophysis 
does  not  close  up,  as  in  the  higher  forms,  but  persists 
throughout  life,  becoming  carried  round  to  the  top  of 
the  head  during  the  embryonic  development  by  differ- 
ential growth  of  neighbouring  parts,  as  has  been  actually 
observed  in  Petromyzon. 


y.' 


wm 


r   M 


■f   ■ 


Pig.  135.  —  Median  sagittal  section  through  the  head  of  young  AmmoccEtes. 
(After  Kui'KFER.) 

The  arrow  indicates  the  extent  to  which  the  hypophysis  has  been  (hypothetically) 
removed  from  the  neighbourhood  of  the  neuropore  (lobus  olfactorius  impar). 

ch.  Notochord.  ec.  Ectoderm,  en.  Endoderm.  ep.  Epiphysis,  hy.  Hypo- 
physial involution,  l.o.  I  -bus  olfactorius  impar.  «.  Nasal  involution,  pm.  Me- 
dian portioi.  of  praemandibular  cavity,  st,  Stomodoeum.  F.M.H.  Primary  fore-, 
mid-,  and  hind-brain. 

In  other  cases,  as,  for  example,  in  the  embryo  of  the 
rabbit,  it  has  been  observed  that  the  hypophysis  actually 
undergoes  a  temporary  fusion  with  the  front  end  of  the 
notochord ;  and  in  all  cases  the  distal  end  of  the  hypophysis 
grows  inwards  as  much  towards  the  notochord  as  towards 
the  infundibulum,  so  that  for  the  embryonic  stages  of  the 
craniate  Vertebrates  it  might  be  said  that  the  relations  of 


HYPOPHYSIS. 


287 


the  hypophysis  to  the  front  end  of  the  notochord  are  as  con- 
stant as  its  relations  to  the  infundibulum.  So  close  is  the 
apparent  relation  of  the  hypophysis  to  the  notochord  that 
at  least  one  zoologist,  Huhrecht,  has  suggested  that  there 
was  originally  a  functional  connexion  between  the  two 
structures. 

Again,  in  the  embryo  of  Acipcnscr,  the  sturgeon,  as 
shown  by  Kupffer,  the  distal  end  of  the  hypophysis 
undergoes  temporary  fusion  with  the  subjacent  wall  of 
the  alimentary  cavity.  In  spite  of  the  extremely  modified 
character  of  the  embryo  of  Acipenser  (the  embryo  being 
flattened  out  like  a  disc  ove-  the  yolk),  Kupffer  regards 
this  fusion  of  the  hypophysis  with  the  endoderm  as  being 
of  great  morphological  significance. 

On  the  contrary,  for  the  reasons  mentioned  above,  I 
would  regard  all  these  fusions  of  the  hypophysis  in  the 
craniate  Vertebrates,  whether  with  the  infundibulum, 
notochord,  or  endoderm,  as  being  of  an  entirely  incidental 
character,  often  due,  perhaps,  to  a  tendency  of  such  con- 
tiguous embryonic  tissues  to  fuse  together. 

I  therefore  suggest  that  :  The  hypophysis  arose  in  con- 
nexion with  a  functional  neuroporc ;  when  the  neuropore 
ceased  to  be  functional,  there  was  no  longer  any  bond  of 
union  between  its  inner  portion,  which  opened  into  the 
cerebral  cavity,  and  its  outer  portion,  which  opened  into  the 
buccal  cavity ;  and  these  two  portions  became  separated  by 
differential  growth  of  the  cerebral  and  body-walls  (cf.  Fig. 
135). 

The  Ascidian  Hypophysis. 

The  development  of  the  hypophysis  in  a  typical  As- 
cidian, its  constriction  from  the  wall  of  the  cerebral 
vesicle  in  the  form  of  a  tube,  and   its  opening  into  the 


'.A: 


Wl 


i    i 


N       1 


lU    -; 


r 


288 


T//E  /'A'  O  TO  CHORDA  TA. 


buccal  cavity,  or  branchial  sac,  have  been  described  above. 
The  most  serious  objection  which  has  been  raised  against 
the  comparison  of  the  hypophysis  of  the  Ascidians  with 
that  of  the  craniate  Vertebrates  is,  that  in  the  former 
the  hypophysis  opens,  not  at  an  ectodermal  surface  into 
the  stomod(Kum,  but  at  an  endodermal  surface  (behind  the 
stomodoeum)  into  the  branchial  sac.  This  is  undoubtedly 
the  case  in  some  Ascidians,  e.g.  Distaplia,  and  probably 
also  in  Clavclina,  etc.  In  Ciona,  however,  as  I  can  state 
after  renewed  study  of  the  question,  it  apparently  opens  at 
first  into  the  buccal  cavity  precisely  in  the  line  of  junction 
between  the  stomodceum  and  the  branchial  sac,  so  that  its 
upper  margin  is  continuous  with  the  stomodoeal  epithelium, 
while  its  lower  margin  is  continuous  with  the  epithelium 
of  the  branchial  sac. 

It  is  probable  that  too  much  stress  has  been  laid  on  the 
question  whether  the  hypophysis  of  the  Ascidians  opens 
at  an  endodermic  or  at  an  ectodermic  surface,  and  that 
thus  the  attention  has  been  diverted  from  the  essential 
fact  that  the  hypophysis  opens  into  the  buccal  tube  at  the 
entrance  to  the  branchial  sac.  In  the  case  of  the  Ascid- 
ians, therefore,  I  should  also  regard  the  fusion  of  the 
hypophysis,  whether  with  the  ectoderm  of  the  stomodceum 
or  with  the  endoderm  of  the  branchial  sac,  as  being  in 
itself  non-essential,  while  the  actual  opening  of  the  hy- 
pophysis (itself  derived  by  constriction  from  the  nerve- 
tube)  into  the  buccal  cavity,  apart  from  the  question  of  an 
ectodermal  or  endodermal  surface,  is  the  essential  point. 


CONCLUSION. 


289 


COXCLUSIOX, 

From  the  facts  that  have  been  recorded  and  the  consid- 
erations that  have  been    ur<,^ed  in  these   pages,  it  would 
ollovv  that  one  of  the  chief  factors  in  the  evolution  of  the 
Vertebrates    has   been    the    concentration    of  the  central 
nervous    system    along    the    dorsal  side    of   the  body  (in 
contrast  to  the  position  of  th.  longitudinal  nerve-cord  of 
Annelids,  etc.,  along  the  ventral  or  locomotor  surface),  and 
Its  conversion  into  a  hollow  tube.     If  it  be  admitted  that 
the  hypophysis  became  evolved  in  connexion  with  a  func 
tional    neuropore,  it    is    obviously  a    structure  which    has 
arisen  within  the  limits  of  the  Vertebrate  phylum,  and  can, 
therefore,  have  no  representative  in  the  typical  Invertebrate 
organisation.     It  has  been  suggested  by  Adam  Sedgwick 
and  VAN  WijHE  that  the  original  function  of  the  central 
canal   of   the   spinal    cord    was    to   promote    the    respira- 
tion (oxygenation)  of   the   tissue  of   the  central    nervous 
system   water  entering  by  the  neuropore,  and  passing  out 
through  the  posterior  neurenteric  canal. 

It  is  not  so  easy  to  form  a  conception  as  to  th<^  prime 
origin  of  the  other  two  cardinal  characteristics  of  a 
Vertebrate  (Chordate);  namely,  gill-slits  and  notochord 

As  to  the  origin  of  gill-slits,  it  has  been  suggested  inde- 
pendently by  Harmer  and  Brooks,  that  they  arose  at  first 
not  so  much  to  perform  the  direct  function  of  respiration 
as  to  carry  away  the  bulk  of  the  water  which  constantly 
entered  the  mouth  with  the  food,  so  as  to  avoid  the  neces- 
sity and    discomfort    of   the  never-ceasing   flow  of  water 
through    the  entire   length   of   the  alimentary  canal.     In 
Cephalodiscus,  for  example,  the  luxuriant  branchial  plumes 
must  be  sufficient  for  the  respiration  of  the  minute  animal 


9t  I 


290 


THE  rKOTOCnOKDATA. 


V  < 


while  the  usefulness  of  the  pair  of  gill-slits,  in  allowing  the 
surplus  water  to  pass  out  of  the  pharynx,  is  evident. 

The  notochord  is  more  difficult  to  explain,  and  the  fact 
of  its  occurrence  in  the  proboscis  of  Balanoglossus  and 
in  the  tail  of  the  Ascidian  tadpole  is  very  puzzling.  The 
mode  of  its  occurrence  in  Balanoglossus  is  undoubtedly 
divergent,  and  not  in  the  direct  line  of  Vertebrate  descent. 
It  is  possible  that  the  notochord  has  not  arisen  through  a 
process  of  elaborate  change  of  function  from  a  pre-existing 
structure,  but  simply  as  a  solidification  of  the  endoderm 
which  was  continued  into  the  caudal  or  post-anal  extension 
of  the  body  to  form  the  axial  support  for  a  locomotor  tail  ; 
while  the  subsequent  extension  of  the  notochord  into  the 
prac-anal  region  of  the  body  is  not  difficult  to  understand. 
The  general  capacity  of  the  endoderm  for  producing 
skeletal  tissue  is  already  present  in  some  of  the  Medusa? 
and  Hydroid  polyps  whose  tentacles  are  stiffened  by  a 
solid  endodermal  axis. 

From  a  purely  morphological  point  of  view  it  now 
seems  as  though  the  prajoral  lobe  and  in  a  lesser  degree, 
perhaps,  the  hypophysis,  would  materially  assist  in  furnish- 
ing the  key  to  a  correct  appreciation  of  the  relationship 
between  the  craniate  Vertebrates,  the  Protochordates, 
and  the  Invertebrates. 

As  we  have  indicated  above,  in  the  formulation  of  the 
Annelid-theory*  no  allowance  has  been  made  for  the  prin- 
ciple of  parallelism  in  evolution  ;  but  it  is  in  possible  to 
doubt  that  this  is  a  very  potent  factor  which  should  always 
be  borne  in  mind  in  estimating  the  genetic  affinity  between 
widely  different  groups  of  animals.  The  closer  the  super- 
ficial resemblance  between  an  Annelid  and  a  Vertebrate 
(in  the  possession  of  somites,  segmental  organs,  etc.)  is 
shown  to  be,  the  more  perfect   appears  the  parallelism 


CONCLUSIOX. 


291 


in    their   evolution    and    the    more    remote    thrir   .•       .• 
affinity.  remote    their   genetic 

•   ,  ,.  vtrtcorates  was  a  free-swmim  n<:  aninnl 

intermediate   n  onranisitir  ,  '  ..f,.,  a     .  '^"'mai 

ind  Am.  w  '^'^'^"'•'^^'^'"  uetween  an  Ascidian  tadpole 

and  Amph.oxus,  possessing-  the  dorsal  mouth,  hypophysi 
and    restricted    notochord  of  the  former;    a;d  t'cmvo' 

rtTe'l^eT'TT'^l  '•"  ^"^  '''''''''  alimenta:    cl; 
ot  the  latter.     The  ultimate  or  primordial  ancestor  of  the 

Vertebrates  would,  on  the  contrary,  be  a  worm-like  Tnimal 
that  of  the  bilateral  ancestors  of  the  Echinoderms. 


NOTES. 

1873-76.     Chall.  Kept.  Zool.  XIX       .ssd      iZ       X^""'"^"- 
Micro.   Sc.    XXVII        ,<!!!,  ""  ""'^I'uUa.     Quarterly  Jo„r. 


I    i      ih!''!!. 


292 


THE    I'h'OTOCI/Oh'DA  TA. 


\h-ik 


\i 


2.  (p.  273.)  On  the  sulycct  of  the  prceoral  lobe  and  the  api- 
cal nervous  system  of  Invertebrates,  see  the  following:  Hai,kour, 
1'".  M.  (Comparative  Mmbryolo^'y.  icSSi.  Vol.  II.  Chap.  12. 
Observations  on  the  Aiui'strat  Form  of  the  Chortiata,  Mkard, 
J.  The  OU  Mouth  an,/  the  New,  A  Stuiiy  in  I'ertehrate  Mor- 
pholoi^y.  .\nat.  An/..  III.  1.SS8.  i)p.  15-24.  Wilson,  \\.  \\. 
The  ./'Ini/iryo/oi^y  of  the  Earthworm.  Jour.  Morph.  III.  18.S9. 
l)p.  387-462.  HArscHKK,  I),  l.ehrhuch  der  Zoolo^ie.  ^d  Licfer- 
ung.  Jena,  1 8g  I.  Wii.i.kv,  A.  On  the  Evolution  of  the  Praoral 
Lohe.     Anat.  Anz.  LX,     1894.     i)p.  329-332. 

3.  (p.  285.)  From  what  has  been  said  in  the  text,  it  is  obvious 
that  the  iiypophysis  of  the  craniate  Vertebrates,  in  becoming 
separated  from  the  neurojKjre,  has  retained  (at  least  in  the  embryo) 
its  primitive  relations  with  the  buccal  cavity,  and,  like  the  latter, 
has  been  made  to  assume  its  present  position  in  consetpience  of 
the  forward  growth  of  the  brain  and  the  ensuing  cranial  llexure. 
In  .\mphioxus,  the  hypophysis  {i.e.  olfactory  pit)  arises  as  an 
ectodermic  involution  immediately  over  the  neuropore,  but  still 
independent  of  the  latter.  In  other  words,  the  neuropore  exists 
in  Amphioxus  for  a  considerable  length  of  time  before  the  hypoph- 
ysis forms  ;  and  this  is  in  accordance  with  what  we  should  expect 
from  the  analogy  of  the  craniate  Vertebrates.  In  the  .Ascidians, 
however,  the  conditions  are  somewhat  different,  and  there  is  at  first 
no  such  obvious  differentiation  between  neuropore  and  hypoph- 
ysis. For  the  simple  Ascidians  {e.i:^.  Ciona)  it  must  at  present 
remain  doubtful  whether  the  increase  in  size  of  the  hypoi)hysis 
takes  place  entirely  by  interstitial  growth,  or  whether  there  is  any 
ingrowth  from  the  wall  of  the  buccal  tube  at  the  lips  of  the  aper- 
ture (dorsal  tubercle)  of  the  hypophysis.  In  any  case  there  are 
not  wanting  indications  in  the  Ascidians  of  a  distinction,  and  even 
separation,  between  the  distal  ])ortion  of  the  hypophysis,  which 
at  first  opens  into  the  cerebral  vesicle,  and  the  proximal  portion, 
which  opens  into  the  buccal  cavity.  In  the  adult,  the  proximal 
portion  of  the  hypophysis  has  the  form  of  a  simple  duct,  opening 
by  the  so-called  dorsal  tubercle  into  the  buccal  cavity,  while  the 
subneural  gland  arises  as  a  proliferation  from  the  ventral  wall  of 
the  distal  portion.  In  Phallusia  /nammi/Iahi,  as  was  discovered 
by  Jui.iN  {Archives  de  Biologie,  II.      1881.     pp.  211-232),  num- 


X07ES. 


^93 


bcrs  o    secondary  tubules  grow  out  from  the  principal  duct  of  the 
hypophysis    and  acquire  ciliated   n.nneldike   openings   into 
pc.r.l,ranchuU  chand,er ;  subse.,uentiy  H^.K...^,.^•  (/>.'   ^W    W 

/^^/-|Y''f'-     "^«-«^-     P-   -45)  found  that  in  this trn/the 
;Iorsal  tubercle,  or  opening  of  the  hypophysis  into  the  buccal  cavitv 
.s  sometunes  absent.     In  Ciona  int^.tinal.  I  have  found  in  you,  J 
nd.vduals  an  ol,hteration  of  the  lunu-n  of  the  hypophysis  beLe 
the  proxunal  and  the  distal  portions.     In  other  cases,  as  in  X; 

ctabsent.  ^'"'''""  "^ ''^'  ^n^^\^\^y^^  may  be  rechued 

On  the  subject  of  the  Ascidian  hypophysis,  the  following  papers 
^hould  .Uso  be  consulted  :  SnK,.o.v,  L.u.x.     Note  on  tk^CiLZ 

«t    ^^JA^//'.--/  Gla,ut     Quarterly  Jour.  Microfsc.  XXVIII 

nt/!;  ,  ^^'y  '^'"'''-     '^'°'''''  J'^"'^"'-     ^'^'^  ^'"  Entroukhn,,:. 
cyclus  dcr Zusammc^csrtztcn  Ascuiicn.     Mitth.  Zool.  Stat.  Neapel 

X.     1893.    PP.5.S4-617.    Mktcalf,  Maynar,,  M.    The  Eyes  a, ui 

I  art  ly.  of  Professor  lirooks's  Monograph  of  the  Genus  Salpa  ) 

4.    (p.  290.)     The  most  complete  presentation  of  the  Annelids- 
theory  ,s   contained    in    the   classical   Mono.rapkie  der  Capitel- 

o  add   that  th,s   monograph  will  command  the  gratitude  and 
admiration  of  zoologists  to  the  end  of  time. 


Wy 


T 


REFERENCES. 


5 
6 


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Carus,  J    Victor.     Geschichte  der  Zoolo^ie.    MUnchen  1872 
ap  des  I-nnctwnswechsels.     Leipzig,  1875 

^.^  J/.;/.r//.,/.     Leipzig,  1874;  4th  Edit.,  189,  ^ 

Lankkstek.   E.    Rav.      Article    " /;v-/./..v^/^,-     £„,,„    j^-. 

'  p4;1er    r'"'"^"^^'  ';  "'°''"^''^'^'  ^^^^^^  London     89"' 

Semper,  Caul.     Die  Ver.vandlschaftsbe.iehun^en  der  ge.lieder 
ten  mere.     Parts  I.  to  III.     VVUrzburg.  1875-76       ""'"  ^'^^''^''- 


I.  and  n. 

ANATOMY   OF  AMPHIOXUS.* 

7  ANDREA'S.    E.   A.      The   BaJuvna  Amphioxus  (prelimin.rv   .. 
;:::rS9f'"^  "^^^'^^^  ^^nivorsi,.  circulars.  ^^T^.  ^ 

8  A.VDREWS,    E.    A.      A.    C7ndcsaif.ed  Acraniatc :   Asv.unctron 
Xiv;  '^-      '^93-     pp.    213-247.     Plate.s  XIIL- 

A™,it:.  ''"'°°"^"'-  "'■  "•'■'"'"""^  »"'■  f--'-  "-U,  o„ 

VII.     I892.     pp.  690-69::.     One  llgiire  in  text. 

295 


296 


HEI'EKENCES. 


Ih:* 


10  Balfour,  F.  M.  A  Preliminary  Account  of  the  Development  of 
the  l.lasmohranch  J-'ishes.     Quarterly  Jour.  Micro.  Sc.  XIV.  N.  S. 

1874.  pp.  323-364.     Plates  13-15. 

Paper  in  which  Halfour  tirst  ])ublishecl  his  discovery  of  the  seg- 
mental origin  of  excretory  tubules.     This  was  made  out  also  in  the 
s^me  yenr  by  Semper  and  Schultz.     (Vide  infra,  Schi/ltc.) 
ir         B.M.Foi'K,  F.  M.      On  the  Orii^in  and  History  of  the  Urino- 
i^enital  Organs  of  Vertebrates.     J',)ur.  of  Anat.   and  Physiol.  A. 

1875.  pp.  17-48.    Eight  figures  in  te.\t.    Amplification  of  his  pre- 
vious work,  with  bibliography  up  to  date. 

12  Balfour,  F.  M.  The  Development  of  Elasmobranch  Fishes. 
Development   of  the   Trunk.     Jour,    of  Anat.  and    Physiol.  XI. 

1876.  pp.  1 28- 1 72.     Plates  5  and  6.     First  account  of  origin  of 
paired  limbs  from  continuous  epiblastic  thickenings. 

13  Balfour,  F.  M.  A  .\fonograph  on  the  Development  of  Elasmo- 
branch fishes.     London,  1878. 

14  Bkddari).  Frank  Evkrs.  On  the  Occurrence  of  Numerous 
NepJiridia  in  the  Same  Segment  in  Certain  Earthworms,  and  on 
the  Relationship  between  the  Excretory  System  in  the  Annelida  and 
in  the  Tlatyhelminths.  Quarterly  Jour.  Micro.  Sc.  XXVIIi.  N.  S. 
1888.  pp.  397-411.  Plates  30-31.  Contains  discovery  of  neph- 
ridial  network  in  Pericha^ta. 

15  Benham.  W.  Blaxlani).  The  Structure  of  the  Pharyngeal 
Bars  of  Amphioxus.  Quarterly  Jour.  Micro.  Sc.  XXXV.  N.  S. 
1893.     pp.  97-118.     Plates  6-7. 

16  Bourne,  Alfred  Giiuis.  Contributions  to  the  Anatomy  of 
the  Hirudinea.  Quarterly  Jour.  Micro.  Sc.  XXIV.  N.  S.  1884. 
pp.  419-506.     Plates  24-34. 

Contains  discovery  of  nephridial  network  in  Pontobdella. 

17  BovERi,  Theodor.  Ueber  die  Niere  des  Amphioxus.  Mlin- 
chener  Medicin.  Wochenschrift.  No.  26.  1890.  Sep.  Abd. 
pp.  1-13.     Two  figures  in  text.     (Preliminary  note.; 

18  Bovi:rl  Theodor.  Die  Niercncaniilchcn  des  Amphioxus.  Ein 
Beit  rag  zur  J^hylogenie  des  Urogenitalsy  stems  der  H'irbelthiere. 
Zoolog.  Jahrbiicher.  Abth.  flir  Morphol.  V.  1892.  pp.  429-510. 
Taf.  31-34  and  five  figures  in  text. 

19  CosT.\.  O.  (jAHRIELE.  Ccnni  zoologici  ossia  descrizionc  som- 
maria  delle  specie  nuo7>e  di  animali  discoperti  in  diverse  contrade 
del  regno  ncW  anno  1834.  Napoli,  1834.  See  also  E'auna  del 
regno  di  Na poll.     1839-50. 

20  CuENOT.  L.  Etudes  sur  le  sang  et  les  glandes  lymphatiques 
dans  la  serie  animate.  Archives  de  zool.  experimentale,  XIX. 
1891.     Amphioxus.     pp.  55-56. 


,1  \ 


/iEfE/^EXCES. 


297 


Notes  absence  of  blood-corpuscles  in  Amphioxus.  Tlu.se 
described  by  previous  authors  must  therefore  require  ai.other  t-x- 
planatic  :i 

21  DonKV,  AXTOX.     Stiuiicn   znr    U,xesc/uc/,tc  ties    lluMZ/ih-r. 
korpers.     IV.  Section  5.     Kntsteliun^  umi  Hedeutun^  dcr  Tlivnn,, 
derSelacJner.     Mitth.  Zool.  Stat.  Neapel.  V.     1884.     nniai-i,-, 
Taf.  8.     Figs.  I  and  2.  '   ' 

22  EisKi,  Hugo.     Die  Se^^mentalorgane  der  Cafiiteiiiden.     Alitth 
Zool.  Stat.  Neapel.  I.     1879.     pp.  93-118.     Taf.  IV. 

Discovery  of  numerous  nephriciia  in  single  segments  and  an- 
astomoses between  successive  nephridia. 

Emery,  Carlo.  Le  specie  del  genere  Fierasfer  net  Golfo  di 
Napoli.  2d  .Monograph  in  the  "  Fauna  und  Flora  des  Golfes  von 
Neapel."     Leipzig,  1880. 

E.MERY,  Carlo,    /.nr  MorpJtoloi^ie  der  Kopfniere  der  Teleostier 
Biologisches   Centralblatt,   I.       1881.      pp.    527-529.      See    also 
Zoologischer  Anzeiger,  VIII.     1885.     pp.  742-744. 

FusARi,    Ro.ME(X     Beitrag   zum   Studium  des  Peripherischen 
AWvensystems  von  Amphioxus  lanceolatus.      Internationale  AIo- 
natsschrift  fiir  Anatomic  und  Physiologic,  VI  .   1889.    PD    i-o-i-io 
Taf.  VII.-VIII.  ■*  ■ 

GooDsiR,  JoHX.  On  the  Anatomy  of  Amphioxus  lanceolatus. 
Transactions  of  the  Royal  Society  of  Edinburgh,  Vol.  XV.  Part  I. 
I 84 I.     pp.  241-263. 

Grexacher,  H.  Beitriige  zur  niihern  Kenntniss  der  Muscu- 
latur  der  Cyclostomen  und  Leptocardier.  {Leptocardia  proposed 
by  Haeckel  as  a  classificatory  name  on  account  of  the  simple 
tubular  "heart"  of  Amphioxus.)  Zeitschr.  fiir  VViss.  Zoolo-ie 
XVII.  1867.  pp.  577-597-  Taf.  XXXVI.  First  isolation" of 
muscle-plates  of  Amphioxus. 

GilNTHER,  Albert.  Synopsis  of  Genus  Branchiostoma.  In 
Report  on  Zool.  Collections  of  H.  M.  S.  Alert.  188  i-S-'  on  ^ii- 
33.     London,  1884. 

Hatschek,   Berthold.     Die  Metamerie  des  Amphioxus  und 

des    Ammoca'tes.     Verb.    Anat.  Gesellschaft,    6th  Versammlun-. 

Wien,  1892.     pp.  137-161.     Eleven  figures  in  text. 
2gbis.    Hatschek,    Bertholi^.      Zur  Metamerie  der    Wirbelthiere 

Anat.  Anz.  VII.     Dec.  1892.     pp.  89-91. 
30         HuxLEV,  T.  H.     Preliminary  Note  upon  the  Brain  and  Skull 

of  Amphioxus   lanceolatus.     Proceedings  of  the   Royal    Society, 

XXIII.     18/4.     pp.  127-132. 

Points  out  that  in  Myxine  and  Ammoccetes  a  velum  is  present 

separating   the   buccal    (stomodcual)  from   the   branchial   cavity. 


23 


24 


25 


26 


27 


28 


29 


':p: 


298 


KEFEKEXCES. 


H';!;. 


ir  I. 


i 


U--i' 


The  resemblance  of  the  uuccal  cavity  and  tentacles  (cirri)  of 
AnimoccL'tes  to  the  corresponding  parts  in  Amphioxus  is  so  close 
that  there  can  hardly  be  any  doubt  the  two  are  homologous.  The 
anterior  end  of  the  nerve-tube  of  Amphioxus  corresponds  to  the 
lamina  tcniiinalis  of  tlie  craniate  Vertebrates. 

31  Huxley,  T.  H.  On  the  Classification  of  the  Animal  Kingdom. 
Journal  of  tile  LinniL-an  Society  (London), XII.  1876.  pp.  199- 
226.     (Read  3d  Dec,  1874.) 

Section  on  *■'■  epica'l,"''  p.  216  et  seq.  Atrial  cavity  of  Amphi- 
oxus and  Ascidians  is  an  epicuil  like  the  opercular  cavity  of  the 
Amphibian  tadpole. 

32  KoLLiKKK,  Aliuckt.  i\'ber  tlas  Geruchsorgan  von  Amphioxus. 
iMliller's  Archiv  fiir  Anat.  Phy«iol.,  etc.  1843.  pp.  ^2-35.  Taf. 
II.  Fig.  5. 

Discovery  of  olfactory  pit  and  first  description  of  the  spermatozoa 
of  Amphioxus. 

33  Koi'1'i:n',  Max.  Bcitr'dge  ziir  vcrglcichenden  Anatomic  des 
Cent  ralncrvensy  stems  der  Wirbelthiere.  Zur  Anatomic  des 
ICidechscngehirns.  Morphologische  Arbeiten  (Schwalbe),  I.  1892. 
pp.  496-515.     Taf.  22-24. 

C\)iitains   discovery  of  giant-fibres  in  caudal  portion  of  spinal 
cord  of  Laccrta  I'iridis. 
74        KoiiL,  K.     Einige  Bemerkitngen  liber  Sinnesorgane  des  Amphi- 
oxus lanceolatits.     Zool.  An/.     1890.     pp.  182-185. 

States  that  sometimes  there  is  a  shallow  olfactory  groove  on  the 
right  side  as  well  as  that  in  the  left.  Such  grooves  are  often  due 
to  artificial  crumpling,  and  the  observation  requires  confirmation. 

35  Kkuken'HKIu;,  C.  Fk.  W.  Zur  Kcnntnis  des  chemischcn  Baues 
■von  Amphioxus  lanceolatus  und  dcr  Ccphalopodcn.  Zool.  Anz. 
1881.  pp.  64-66.  See  also  Hoim'e-Seylek's  reply,  pp.  185-187. 
Compare  also  Cukn'OT  (supra). 

36  Kui'FFER,  Carl  vox.  Studicn  zur  verglcichende  Entwick- 
lungsgcschichte  des  Kopfcs  der  Kraniotcn,  /.  Die  Entiuicklung  des 
Kopfes  von  Acipeuscr  sturio  an  Mcdiansclinitten  untersucht.  95 
pp.     8  .     9  Tafeln.     Munchen  und  Leipzig,  1893. 

Contains  also  a  chapter  on  brain  of  Amphioxus,  with  figures. 

37  La  X(;i:u  HAN'S,  Pai'I  .  Zur  Anatomic  des  Amphioxus  lanceolatus. 
Arcliiv  fiir  mikroskopische  Anatomic,  XII.  1876.  pp.  290-348. 
Taf.  X1I.-XV^ 

Standard  work  on  the  histology  of  Amphioxus. 

38  Lankester,  E.  Ray.  On  Some  lVc%i.<  Points  in  the  Structure  of 
Amphioxus  and  their  Hearing  on  the  Morphology  of  I  'ertebrata. 
Quarterly  Jour.  Micro.  Sc.  X\'.  N.  S.     1875.     pp.  257-267. 


J^EFEKENCES. 


299 


39 


40 


41 


45 


46 


Lankester,  E.  Rav.    Contributions  to  the  Knau^ledge  of  4>nthi 
-:;;j^atus,yarren,     lb.,  Vol.  XXIX.     ,889.     l'^. 

LwoKK.  Basilius.     Uber  den   Zusannnenhang  von  Markrohr 
nnU   CJunuia    beun    A,nphio.rus    nnU    ahnliclj Verhi^^^^ti 
AnneUde,:     Zeitschrift  fur  wiss.  Zoologie.  Bd.  65         80         t 
299-308.     Taf.  XVII.  ^         ^^-     ^'I'- 

Describes  those  supporting  fibres  of  the  .spinal  cord  of  AmDhi 
oxus  wh.ch  descend  in  successive  paired  groups  to  tl  e  notodr^^," 
sheath  and  penetrate  the  latter  in  order  to    nsert  themselve  Tn 

sheath  of  Amphioxus.  through  which  the  ventral  supportin.r  fibres 
pass,  were  first  observed    by  Wilhelm  MOllek  in   rS;,'"'    (W 

LWOE.  (88).     I^Uter^n^:;^^:;^,^:^^-;:^^ 
relating  to  structure  of  notochord.  '        i'terature 

Mavek    Paul.     Uber  die  Entwicklung  des  Herzens  und  der 
iNtapej.  VII.     1887.     pp.  338-370.     Taf.  11-12. 

VTit  f''?''  ,^c '''""'•     -'■'''^'''"'^  '''^^'"  ^'■'"  Kiirperbau  der  Anneliden 
Alitth.  Zool.  Stat.  Neapel.  VII.     1887.     nn   ,q.  7^,      ^>'"^Men. 

,2  bis      M0KEAU.CAMI.LE.      AV^W././^iL':  ;  Jl;"/^^^ 

:^r^^'T'T;  '""•  ^^^^-  ^^'^-  ^^-3^  ^^ 

1075-     22  pp.     One  plate.  •* 

^r,,,.s,erinr,Mure.    76  pp.    Five  pla...    4.     Ldp^ 
iMOLLER  W,LHEI.„.     Ueber  das  UrogenitahysU,,,  d,s  A,„t,,i 

xt;.;;,'!!  '""  ^"'*'"'-  ■'''•  ^^■"-  '^'"'"■*-  p^-  '-33.  • 

This    is   the    important   work   in   ivhich   the  pronephros  and 

Kead  at  the  konigl.  Akademie,  1841 

Naxsex,  Friotjok.  The  Structure  and  Con,l,i„ation  of  the  His- 
tologual  J^,nents  of  the  Central  Nervous  Syste>n.  IJer.^ns 
Museums  Aarsberetning  for  1886.     Bergen,  18S7." 


43 


44 


;03 


KE/'EKKXCES. 


48 


49 


50 


47  OwsjANNiKow,  Philip.  L/chcr  lias  Centralnervensystcm  dcs 
Ainphioxus  laiiceoliitiis.  lUillctin  de  TAcad.  imp.  des  Sciences  de 
St.  Fcteisbourg,  Tome  XII.  1868.  pp.  287-302,  with  one  plate. 
Also  in  Mclange.s  Biologi(|ucs,  T.  VI.     pp.  427-450. 

Introduced  a  metliod  of  maceration  by  which  he  was  able  to 
shake  out  the  central  nervous  system  and  thus  isolate  it  from  the 
body.  In  this  way  he  was  able  .0  correct  the  erroneous  descrip- 
tions of  (ic  (Jiialri'fafi^es  and  others  (who  stated  that  there  were 
ganglionic  enlargements  in  the  spinal  cord),  and  to  discover  the 
alternate  arrangement  of  the  spinal  nerves. 

Platt,  Julia  W.  Fibres  connecting  the  Central  Nervous  System 
and  Chorda  in  Amphioxns.  Anat.  Anz.  VII.  1892.  pp.  282- 
2S4.     Tliree  figures  in  text. 

PoLLAKi),  E.  C.  A  New  Sporozo'dn  in  Atnphioxiis.  Quarterly 
Jour.  .Micro.  Sc.  XXXIV.  N.  S.  1893.  pp.  311-316.  Plate 
XXIX. 

Unicellular  parasites  in  intestinal  epithelium. 
49  to-.   PdiciiKT,  (ii:oRGF,.s.     On  the  Laminar  Tisstte  of  Amphioxns. 
Quarterly  Jour.  Micro.  Sc.  XX.  N.  S.    pp.  421-430.    Plate  XXIX. 

DE  QjATKEFACKS,  Au.MANi).  Mcmoire  sur  le  systime  nerTenx 
et  sur  riiistologie  dii  Hranchiostome  on  Ainphioxus.  Annales  des 
sciences  nat.  Zoologie.  3d  series.  IV.  1845.  pp.  197-248. 
Plates  10-13. 

First  observation  of  passage  of  ova  through  atriopore ;  and 
discovery  of  the  peripheral  ganglion-cells  in  connexion  with  the 
cranial  nerves. 

Kathke.  Heixrich.  Bemerk-ingen  ilber  den  Ban  des  Amphi- 
oxns lijiiceolatus,  eincs  Fisches  aus  der  Ordnung  der  Cyclostomen. 
Kcinigsberg.  1841.     4°.     pp.  1-38.     One  plate. 

Retzh'S,  Gustav.  Znr  Kenntniss  des  centralen  Nervensystems 
von  Amphioxns  lanceolatus.  Biologische  Untersuchungen.  Neue 
Folge  II.  pp.  29-46.  Taf.  XI.-XIV.  Stockholm,  1890. 
c2  his.  Retzius,  Gustav.  Das  hintere  Ende  des  Riickenmarks  nnd 
scin  I 'er  halt  en  zur  Chorda  dor  sal  is  bei  Amphioxus  lanceolatus. 
Verh.  Biol.  Vereins.  (Hiologiska  Foreningens  Forhandlingar.) 
Stockholm.     Bd.  IV.     pp.  10-15.     9  ^S^-     i^9i- 

RdHDE.  Emil.  Histologische  Untersuchungen  iiber  das  Nerven- 
svstem  von  Amphioxus  lanceolatus.  In  Anton  Schneider's  Zoo- 
logische  Beitrjige.  Bd.  II.,  Heft  2.  Breslau,  1888.  pp.  169-2 11. 
Plates  XV.-XVI. 

Standard  work  on  the  central  nervous  system  of  Amphioxus. 

RoiK^N.  JosEE  Victor.  Untersuchungen  iiber  Amphioxus 
lanceolatus.     Fin  Beitrag  zur  7<t.rgleichenden  Anatviie  der  \\  'ir- 


51 


52 


53 


54 


J^EJ-EKEXCES. 


301 


55 


56 


57 


58 


59 

60 

61 

62 


belt/iiere.  In  Denkschriften  der  .Math.-Naturwiss.  Classo  iler  kais 
Akacl.  der  Wissenschaften.  Bd.  XLV.  VVien,  1882.  641,,,  .- 
Six  plates.  t  1  1       -+  • 

Relates  chiefly  to  nervous  system.  Describes  also  the  sniootii 
muscle-lilM-es  in  wall  of  pharynx,  etc.  Finds  that  the  niajoritv  of 
sensory  nerve-tibres  to  the  skin  end  freely  between  the  cells  of' the 
ectoderm  in  bush-like  ramifications.  For  the  rest,  -  'e  Naxskn 
KoiiDK,  Rktzil's.  and  Fl'saui. 

Roi.i'M,  W.  C->itc,-sin/nnt,i::cn  uber  den  Pnui  dcs  Amphioxus 
laiiccolatiis.  Morphologisches  Jahrbuch,  II.  1S76.  pp.  87-164. 
Taf.  V'.-V'II. ;  also  figures  in  text. 

Ri:cKi:RT,  Jc.HANNKs.  Entwkkduv^  der  Excretionsor^ane. 
I-rgcbnisse  der  Anatomie  und  Entwicklungsfreschichte  (Merkel 
und  IJonnet),  I.  1891.  pp.  606-695.  Includes  an  extensive  bibli- 
ograp'.iy. 

SciiNEiDKH,  AxTox.  Eeitnige  zur  vcri^lcichcndcn  Anatomie 
und  Entwicklnni^s,^eschichtc  der  IVir  belt  lucre.  /.  Amphiovns 
lanceolatus.     pp.  3-31.     Taf.  XIV.-XVI.     40.     lierlin,  1879. 

SCHULTZ,  Alkxaxdicr.  Zur  Entwickelitni^si^escliichtc  des  Sehi- 
chujeies.  Archiv.  fur  Mikr.  Anat.  XI.  ,875.  pp.  569-380. 
Taf.  34. 

Prelii-inary  notes  of  both  Semper  and  Scludtz.  regardin.^  the 
segmental  origin  of  the  excretory  tulxiles,  were  published  in  the 
Centralblatt  tiir  Medicinische  VVissenschaft,  1874. 

Skmox,  Richard.  Studien  Uber  den  liaupian  des  Urogenital, 
systems  der  Wirbelthiere ;  dari^elegt  an  der  Entiuiekelnn^  dte^es 
ih-^ansystems  bei  Ichthyophis  glutinosus.  Jenaisdie  Zeiischrift, 
XXVI.  1891.     pp.  89-203.     Taf.  I. -XI V^ 

Spex(;i: L.  J .  W.  neitra,i:;=.ur  Kcnntniss  der  Kiemen  des  Ampiii- 
ox-iis.  Zool.  Jahrbiicher.  Abth.  fiir  Morphol.  IV.  i8qo.  Dn-;7- 
296.     Taf.  17-18.  li  •  -3/ 

SPEXCiKL,  J.  W.  lknham\^  lOitik  meiner  Am^aben  'uber  die 
kiemen  des  Amphioxus.     Anat.  Anz.  VIII.     1893.     pp.  76-'-765 

Stieda,   Ll'I.U'h;.      Studien  uber  den  Amphioxus  lameolatu, 
Mem.  de  TAcad.  Impcriale  des  Sciences  de  St.  I'etersbourg,  7th 
series,  Vol.  XIX.     No.  7.     70  pp.     Four  plates.     ,873. 

Contains  some  good  observations  on  the  central  nervous  svstem 
First  to  show  that  the  split-like  structure  above  central  canal  diil 
not  correspond  to  the  posterior  fissure  of  the  vertebrate  spinal  cord. 
but  was  a  portion  of  the  original  central  canal  itself,  the  lumen  of 
which  had  been  partially  obliterated  in-  approximation  of  its  walls 
First  Identification  of  ventral  (motor)  roots  of  spinal  nerves  in 
Amphioxus. 


302 


KEJ'IIJ^K.VCES. 


63  Thacher,  Jamks  K.  Median  and  Paired  Fins  ;  a  Contribution 
to  the  History  of  Vertebrate  Limbs.  Transactions  Connecticut 
Academy,  III.    No.  7.     1877.     pp.  281-310.     Plates  49-60. 

64  Wkiss,  F.  Eknkst.  Jixcretory  Tubules  in  Amphioxns  lanceolatus. 
Quarterly  Jour,  of  Micro.  So.  XXXI.  N.  S.  1890.  pp.  489-497. 
Plates  34-35. 

65  VAN  WijUK,  J.  \V.  Ueber  Amphioxus.  Anat.  An/..  VIII.  1893. 
pp.  152-172. 

66  \AN  Wijiu;,  J.  W.  Die  Kopf region  der  Cranioten  beim  Amphi- 
oxus, nebst  Bemerkungen  'liber  die  IVirbeltheorie  des  Schiidels. 
Anat.  Anz.  IV.     1889.     pp.  558-566. 

67  \'AN'  Wijm:.  J.  VV.  Ueber  die  Mesodermsegmente  des  Ruvipfes 
und  die  E)it7vicklung des  Excretionssy stems  bei  Selachiern.  Archiv. 
f.  Mikr.  Anat.  XXXIII.     1889.     pp.  461-516.     Taf.  30-32. 

68  Wii.i.KV.  AkTHL'K.  Report  on  a  Collection  of  Amphioxus,  made 
by  Professor  A.  C.  Haddon,  in  Torres  Straits,  1888-89.  Quarterly 
Jour.  Micro.  Sc.  XXXV.  N.  S.  January,  1894.  pp.  361-371.  One 
figure  in  text. 

Branchiostoma  cultellum      Peters. 


III. 
DEVELOPMENT  OF  AMPHIOXUS. 

69  Ayers,  Howard.     Bdellostoma  Dombeyi,Lac.     A  Study  from 

the  Hopkins  Marine  Laboratory .  Biological  Lectures,  Marine 
Biological  Laboratory,  Woods  Holl.  1893.  No.  VII.  Boston, 
1894. 

69  bis.  Bert,  Paul.  On  the  Anatomy  and  Physiology  of  Amphioxus. 
Annals  and  Mag.  of  Nat.  Hist.,  3d  Series.  Vol.  XX.  1867. 
pp.  302-304.  (Translated  from  Comptes  Rendus.  Aug.  26th, 
1867.     pp.  364-367.) 

Breeding  season  of  Amphioxus  at  Arcachon  is  from  March  to 
May.  Was  the  first  to  observe  the  ejection  of  the  sperm  through 
the  atriopore.  Calls  attention  to  remarkable  lack  of  regenerative 
power  in  Amjjhioxus.  Individuals  cut  in  two  will  li\e  for  several 
days,  but  will  not  regenerate.  ''  If  the  extremity  of  the  body  of 
an  Amphioxus  be  cut  off,  the  wound  does  not  cicatrize  ;  on  the 
contrary,  the  tissues  become  gradually  disintegrated.  I  have 
seen  animals,  with  only  the  tail  mutilated,  become  gradually 
eaten  away  up  to  the  middle  of  the  branchial  region,  and  live 
thus  without  any  intestines,  without  abdominal  walls,  and  without 
branchia;  for  several  days,"    These  observations  of  Paul  Bert  are 


L.?-'< 


I^I-FF.K/uVCES. 


303 


70 


71 


72 


73 


74 


75 


76 


77 


78 


79 


v^w  o?  1'"'^'  ^«"«™=^»-".  and  should   he   h.,r„e  in  n,ind  in 
r  red   ;',;^"'''^"'-^""''''>-  '•^••^'-^^'nuive  pcnver  uluch  Wilson  dis- 
co, ered  in  the  segmentation  stages  of  the  embryo 

BOVKKI.  Thkoi,(,k.     6%7-  Uu-  lUUun^.statte  dcr  Gcschlechts- 

-.^    Anat   Anz.  VII.     ,893.     pp.  ,70-8,.     Twelve  figures 

korpers.  III.     Die  l-Mchun^  und  ncdcutuu.^  Mr  Ilvpopi.vsis 
be,  Petro.nyaon  Planeri.     Mitth.  Zool.  Stat.  Neapeh  iv'.     ^^ 

^OH,  .Imp/iioxiis  und  I itiiicateii.     Ih.  \\.     ,885 

Dohrn  lays  unnecessary  stress  upon  the  tact  that  often  in 
transverse  section,  especially  in  the  anterior  region  of  the 
Pharvn.x  the  endostyle  of  Amphioxus  projects  up  into  the  an  i  v 
of  the  pharynx  ,n  the  form  of  a  convex  lens-shaped  ridge.  Tlii's 
.s  merely  due  to  the  muscular  contraction  of  the  pharvtix.  uhich 
almost  invariably  takes  place  v.hen  Amphioxus  is  placed  in  a 
killing  reagent.  It  is,  therefore,  not  an  anatomical  feature  of 
any  significance. 

DoHKx.  AXTOX.     Studien,XII.     Thyreoidea  und  Hypobran- 
clualnnne,  ^pntM.ack  und  Pseudoln-ancniainnne  belUZ, 
Ammocwtes  und  Tunikaten.     lb.  VII       1887 

DOHK.X,  AXTO.X.     Studien,  XIII.     Uber  Xerven  und  Geflis^e 
be,  Am,„oca'tes  ,a,d  Petromyzon  Planer,:     lb.  VI 1 1       ,888 

Pkokik,..     August.       /•:nt,a,a-el,,n,^rs^^resc/,,-c/M'    des     Kopfe. 

^^\      x'sof"""    "^    ^^"t-ckelungsgesch    (Merkel    und 
linnet)     I.     189,.     pp.  561-605.     Eleven  figures. 
Includes  an  extensive  bibliography 

.r!!-^'''^,"''/''''™?"-    ^'"'^'"'  "''"-  E^^^^^^i^klnngdes  A,nphi. 
Zeplatf  "'^  ''^-    '•    ^°°'-    ^-^^--     ^^^-'««'-     ««i'P. 

Hatschek,     Berthoi.,,.      Mittheilungen     uher     A,nphiovns 
Zoolog.scherAnzeiger,VII.     1884.     Pp.  Si7-S-'o 

^m!^;!::iz.:^''  '^'-''"^ '' ''---'  ^~-^-  p-o-1 

Hatschek   Berthold.     Uber  den  Scluchtenbau  von  A,npin. 
--.     Anat.An..in.     ,888.     pp.  663-667.     Five  figures.  "^ 
Origin  of  sclerotome,  etc. 

e,nb,yos.     Anat.  Anz.  III.     1888.     pp.  445-467 

One  of  the  first  to  bring  forward  definite  embrvological  ficts  to 
prove  tlK^t  ^,e  anterior  (pne-auditory )  head-cavities  o^^•Ax  W  ,  e 
(UeberdieMesodermsegmente.etc,  des  .Selacliierkopfes.  Amster- 


304 


KKj-7-:KJLyc/-:s. 


]m 


■u 


80 


81 


83 


84 


85 


86 


(lam,  1882)  arc  not  lujinudynamous  with  tlie  triii;  somites.  Ik- 
was  followcil  in  this  respect  by  Kaim,  (Tliuoric  cles  Mesoderms. 
Morplioiogisches  Jahrinich,  XV.     1889). 

KoKscjiiiii/r,  K.,  und  Hkiijkk,  K.  Lchrbiuli  der  verghiclien- 
(it'/t  Enhk.>ickluni:;sgeschklitc  tier  wirbcllosen  Tliicye.  3d  Melt. 
Jena,  1893. 

K()WAi.i:vsKY.  Am:xa\I)F.K.  linhi'icklmi'^si^esiliichtc  ifrs  .1///- 
phinxiis  liUUcoUitus.  Mt'in.  cle  i'Acad.  I  mi),  t'.os  Sciences  de  St. 
I'cHersljoury.  \11.  Series.  T.  Xl.  No.  4.  1867.  Three 
plates. 

KowAi.icx'SKV,  Ai.KXANDKK.  ll'eiterc  Stitdien  I'lhcr  die  F.tit- 
7c>hkli(iii^si:[i'sc/iic/ite  des  .Iniplnoxns  lamcolatits,  iicbst  eiiieiii 
licit nii^e  ziir  J/oi/io!oi;;ie  dcs  A'cr^'t'iisysteiiis  dtr  W'uriiier  11  ltd 
M'irbcltliicrc.  Arch.  f.  Mil<r.  Anat.  XIII.  1877.  pi).  181-204. 
Two  phitcs. 

Amonj^  the  definite  discoveries  communicated  l)y  Kowalcvsky 
in  these  two  memoirs  may  Ije  mentitined  tiie  following :  General 
features  of  segmentation  and  gastrulation,  origin  of  mesoderm 
from  archenteric  pouches,  unifjue  method  of  formation  of 
nerve-tul)c  (see  text),  origin  of  notochord,  neurenteric  canal, 
asymmetrical  origin  of  gill-slits  and  mouth,  and  in  part  the 
metamorphosis. 

Kui'i-i-i;i<,  Cart.  \()n.  Die  Enhi<icklinii^  von  Petroiiiyzon 
Planeri.  Arch.  f.  Mikr.  Anat.  XXXV.  1890.  pp.  469-558. 
Six  plates. 

Origin  of  head-cavities,  hypophysis,  etc 

Kl'I'KFIok.  CAur.  \'()N.  Die  I-lntwicklitii;^  dcr  Kopfnerven  der 
Vertebratcn.  Verhandl.  Anat.  (lesellschaft  in  .Munchen.  1S91. 
pp.  22-55.  Eleven  figures.  (Erganzungsheft  zum  Anat.  Anz. 
VI.     1891.) 

Amnioc(etes  (see  Fig.  92  in  text). 

Kl'I'1"I'i:k,  Caki.  vn.  Stiidien  ziir  vcrgleichende  Kiitivick- 
litngsgescliichtc  des  Kopfex  der  Kranioten  I.  Die  Eiitioicklitng 
des  Kopfes  7ion  .Icipenser  stiirio  an  Mediattsc/mitten  tmtersueJd. 
pp.  95.  Nine  plates.  Seven  figures  in  text.  Miinchen  and 
Leipzig  1893. 

Important  contribution  to  the  delimitation  of  the  wall  of  the 
brain.  On  page  84  is  a  reconstruction  of  head-cavities  of  Am- 
mocoetes  (see  Fig.  72).  Figs.  21  and  22  in  the  plates  repre- 
sent cerebral  vesicle  of  Amphioxus.     (Cf.  F"ig.  51.) 

Lankkstek,  E.  Rav.  and  Wiijj:v.  A.  The  Development  of 
the  Atrial  Chamber  of  Amphioxus.  Quarterly  Jour.  Micro.  Sc. 
XXXI.     1890.     pp.  445-466.     Four  plates. 


^•/■/■■/':a'£xc/,s. 


S7 


30s 


88 


89 


90 
91 


92 


93 


Lf.UCKAKT,    RuOOM.H.    unci     PAriKXSTRCHFK     A.  FX         /'. 

XVIII.  •'"''•     '^5'^-     PP-55«-569.     Taf. 

Ucscripticn   of   larva,   of  Amphioxus  taken    off    Fr  r     ,      . 
Drew  attentiun  to  larval  asvmmc  rv    uid  t     n?  ^'^''■««''-i"d- 

brain-vcntriclc  (cerebral  vjsid.^      ,        ,  ^'•^'^''-'nce  of  ,he 

early  deveh-pn^n^Udir^^^         f  ^"-  '>^  ^--'-l^^e  of 

i-'-ia"y  i>-o.-a.  pit.  :rrr  ^i;::;;;:;..:^  r  ""'^^ 

Latter  applies  also  to  Schult.e's  observations  "'■''"''• 

.  "i^.  •  r '^Lttt^::::  r^  ttir^  '"■  ^"-^^ 

1'^-     '«9i.     pp.  483-50..     otpltte  "'■    "'"""•    ^^^^'''^■'• 

pp.  7^9-744.  ;^:S;J'"''"^'^^'^^^^^  '«93. 

Notes  al)sence  of  mesodermal  "pole-cells  "     From  r 

no,ocl,„„l   and    ^^J^^^ ^^^Z' ''7  "''"" 
^./»/,7-«  round  dorsal  lip  „f  b  a,'S«l      H  *  '"  ,   °"'  ""^ 

m^odenn  are  essential,,'  deriv^l  "ree.odt:"  ""'"""'  """ 
_^M,vusH,u..,  A.   M,,..,,.     r„-,.W.  A>,,v,;^^.     ,„„,„„ 

First  accurate  description  of  larva  of  Amphioxus   n    ,v,      , 

peculiar,  in  ,„a"  ,hl  „  '^e  U       irr:.;'  '",!  ^'"r"''  ""» 
wall,  placed  one  above  tl,e  o  l«,        n  ,hn      '"     "  '*''"""'™' 

r^-:---— £;^re= 
;..„d.be,oun,..n.ne,t:^iu^r'ti:it;::; 

MULLER,  WiLHELM.     Uebcr  die  Hypobranchialnnne  der  Tu,u 
katen  und  deren   Vorhaudensein  bei  A,np,uo.us  ,  cZ 

sto.,en.  Jena,sche  Zeitschrift  f.  Naturwiss.  VII.   1873.  j^;  3''  ," 

the  Vertebrate  Head     Anat.  Anz.  VI.     1891.     pp  ->-,   4-         "^ 


3o6 


KKJEKE.\CI:S. 


94  Raul,  Caki..  Cher  die  I yijferenzieruni^^des  Mesoderms.  Anat. 
Anz.  III.     188.S.     pp.  ^167-673.     Kij^Iit  fij^urcs. 

Discovervof  tlio  sclc'rotonic-divcrliciihiin  in  cinl)ivt)i)f  I'ristiiirus. 

95  Ku  1;,  Hk.nkv  J.  (fhservations  upon  the  Hahits,  Structure,  and 
Development  of  Ampliioxus  Linceolatus.  American  Nat.  XI V. 
1880.     pp.  171-210.     I'latcs  14  and  15. 

.Author  was  the  first  to  find  Amphio.xus  in  Chesapeake  Hay. 
Witli  ifi^aril  to  development,  lie  j^ives  some  fairly  j^ood  tij^ures  of 
larv.i'.  and  ohserved  some  of  the  more  obvious  features  of  the 
metamorphosis,  as  alreatly  desciloed  by  Kowalevsky. 

96  KicKiCKT,  JonANNKS.  Ueber  der  Kntstehung  der  Krcretions- 
oriidne  />ei  Se/<i(///ern.  Arch,  fiir  Anat.  u.  Physiol.  (Anatomische 
Abtheilunji).     18S8.     pp.  205-278.     Three  plates. 

Contains  also  the  discovery  of  segmentiil  orij^in  of  j^onads. 

97  S(  iiNi:ii)i".u,  Anton.  Ih'ii'iii^e  zur  Teri^leichenden  Anatomie 
und  /-'.ntiL'tckluni^Si^esc/iic/ite  der  H'irhelt/iiere,  11.  Anatomie  und 
Ent^vickl.  von  Vetromyzon  und  Ammoca'tes.  4'  .  Ten  plates. 
Berlin,  1879. 

Figure  of  the  ciliated  grooves  in  pharynx  of  Ammoccctes,  at 
page  84. 

98  ScHUl-TZK.  Max.  Heobaclduuf^ juniper  Kxemplare  von  Amphi- 
oxus.     Zeit.  f.  VViss.  Zool.  III.       1851-2.     pp.  416-419. 

Two  larvx'  from  Heligoland.  Good  description  of  structure  of 
notochord. 

99  VAN  WijHK,  J.  W.  Ueber  Avtphioxus.  Anat.  Anz.  V'lII. 
1893.     pp.  152-172. 

100        WiiXEY,  A.     On  the  Development  of  the  Atrial  Chamber  of 

Amphioxus.     (Preliminary  communication.)     Proceedings  of  the 

Royal  Society,  XLVIII.     1890.     pp.  80-89. 
loi         WiM.KV,  A.     The  Later  Lar7>al  Development  of  AmpJiioxus. 

Quarterly  Jour.  Micro.  Sc.  XXXII.     1891.    pp.  183-234.    Three 

plates. 

102  Wilson,  Edmiino  R.  On  i\fuUiple  and  Partial  Devolopment 
in  Amphioxus.  Anat.  Anz.  VII.  1892.  pp.  732-740.  Eleven 
figures. 

In  this  and  the  following  more  detailed  paper,  the  author 
describes  and  interprets  a  remarkal)le  series  of  e.vperiments  on 
the  artificial  production  of  twins  and  dwarfs.  Besides  this,  there 
are  many  important  observations  on  the  normal  cleavage  of  the 

egg- 

103  Wilson,  Edmund  B.  Amphioxus  and  the  Mosaic  Theory  of 
Development .  Journal  of  Morphology.  VIII.  1893.  pp.  579- 
638.     Ten  plates. 


f^'F-FEKEXC/'S. 


307 


IQ|       ZiEGi.ER,  H.  Ernst     /;,.,  r,-^...       j 

PP.37.S-400.     One  plate       ^^^'"^  *■  •^"•^'•- Anat.  XXXII.     ,888. 
Indcpculcnt  discovery  of  sclerotome-diverticulum.    (Sec  Rabl.; 


IV. 


ASCIDIANS. 

Forbibliography  relating  to  the  As(i,Ii-,n.    •      n    c 

expedition-  Parts    I  -  ;"'"'"'  f '^'^'  ''"■'  "  Challenger" 

HruDER.  ..  Lerhuuch   de  r  v  ..'1"    .       ,'"      "'^"    J-'oKsci.Ki.T    „nd 

cler  wirbellosen  T       e  "  „    f  7;'"'7'^'"   ^^twicklungsgeschichte 

ntit  ill.     Jena,  1893. 


V. 


PROTOCHORDATES,  ETC. 


105 
106 


107 


\qI 


109 


IIO 


1884-86.  -^  ''°-  ^^-     ^o's-  ^^XIV.-XXVI. 

Monograph  of  the  Genus   Sa  oa       ^^     t     l"'     ^^''''  ''•  "^ 
Baltimore,  1893.  "^  "°^'""'  ^''"^•^'-sity. 

320-334.  >vnat.  Anz.  IX.    1894.     pp.  152-155  and 

Relates  to  neuropore  of  craniate  Vertebrates      Anti  „     , 

lobus  olfactorius  impar  of  KuptTer  the    .!.  ''"'  ''^'-' 


3o8 


REFERENCES. 


%t 


111  DoHRN,  Anton.  Studien  zur  Urgcschtcnte  des  Wirbelthier- 
kdri>ers,  /.  I)er  Miind  der  Knochenfische.  Mitth.  Zool.  Stat. 
Neapel.  III.     1881-2.     pp.  253-263. 

112  FlKLij,  Gkokgk  W.  The  Larva  of  Asterias  vtdgaris. 
Quarterly  Jour.  Micro.  Sc.  XXXIV.     1892.     pp.  105-128. 

113  FowLKK,  G.  HriRBKKT.  The  Morphology  of  Rhahdopleura 
Normaiti .llbiian.  Festschrift  flir  Rudolf  Leuckart.  pp.  293-297. 
Leipzig.  1892. 

114  Hakmkk,  S.  F.     See  AFIntcsh. 

115  Hkkdman,  W.  A.  Article  ''  Tunicata.''''  Ency.  Rrit.  9th  ed., 
republished  in  "Zoological  Articles"  by  Lankester.  etc. 

116  HuiJKKCHT,  A.  A.  W.  Article  '•'■  Nemcrtinesy  Ency.  Brit. 
9th  ed.,  republished  in  "Zoological  Articles"  by  Lankester.  etc. 

\\6bis.    HUBKLCHT.    A.    A.    W.      On    the  Ancestral   Form   of  the 
Chordata.    Quarterly  Jour.  Micro.  Sc.  XXIII.    1883.   pp.  349-368. 
For  later  works  on  this  subject  see  Notes  to  Chap.  V. 

117  Kui'KKKK,  C.  vox.  Fltdivickcluugsgeschichte  des  Kopfes.  In 
Merkeland  Bonnet's  Ergcbnisse  der  Anatomic  und  Entwickelungs- 
geschichte.  II.     1893.     pp.  501-564. 

118  Lani;.  Aknoli).  Zinn  Verst'dndnis  der  Organisation  von 
CeplialodiscHS  dodecalophus  MVnt,  Jenaische  Zeitschrift  f. 
Xaturwiss.  XXV.     1891. 

119  Lan(;,  Arnold.  Uebcr  den  Kinfluss  der  festsitzenden  Lebens- 
wcise  aiif  die  Thiere.     Jena.  1888. 

120  Lankkstkr,  E.  R.vv.  Degeneration  :  a  Chapter  in  Darwinism. 
Nature  Series.  London.  1880.  Republished  in  "  The  Advance- 
ment of  Science ;  Occasional  Essays  and  Addresses."  London. 
1890. 

121  Lanki:sti:r,  E.  Ray.  A  Contribution  to  the  /knowledge  of 
Rhahdopleura.  Quarterly  Jour.  I\Iicro.  Sc.  XXI V.  1884.  pp. 
622-647. 

122  M.vcBride,  E.  W.  The  Organogeny  of  Asterina  Gibbosa. 
Proceedings  Royal  Society.    Vol.  54.     1893.     pp.  431-436. 

123  M'Intosm.  Wu.liam  C.  Report  on  Cephalodiscus  dodecalo- 
phus, }r  Intosh.  "Challenger"  Reports.  Zoology.XX.  1887. 
With  Appendix  by  S.  F.  Har.mer. 

1 24  M()R(;an.  T.  H.  The  Growth  and  Metamorphosis  of  Tnmaria. 
Jour.  Morph.  V.     1891.     pp.  407-458. 

125  M()R(;an,  T.  H.  The  Development  of  Balanoglossus.  Jour. 
Morph.  IX.      1894.     pp.  1-86. 

126  Platt.  Julia  B.  Further  Contribution  to  the  Morphology  of 
the  Vertebrate  Head.     Anat.  Anz.  VI.     1891.     pp.  251-265. 

Describes  the  double  origin  of  mouth  in  Batrachus. 


/iEFEJ^EXCES. 


309 


Observations  on  the  Development  of  the  Head 
Quarterly  Jour.  Micro.   Sc.   XXXV.     1894. 


127  Pollard.  H.  B. 
in  Gobius  capita. 
PP-  33:;-352. 

127  bis.    PoLLAKD.  H.  B.     The  "  Cirrhostomiar  Ori.rn  of  the  Head 
nt  Vertebrates.     Anat.  Anz.  IX.     ,894.     pp.  349-359 

Rabl-Ruckhard,    H.      Der  Lobus    Otfactorlsfrnpar    der 
Selach^er.     Anat.  Anz.  VIII.     ,893.     pp.  728-731. 

Sedgwick,  Adam.     The  Or.^inal  Function  of  the  Canal  of  the 
Central  Nervous  System  of  Vertebrata.      Studies  from  Morph 
Lab.  Cambridge,  II.     1884.     pp.  160-164. 

Sedgwick,   Adam.      Notes   on  Elasmobranch  Development 
Quarterly  Jour.  Micro.  Sc.  XXXIII.     1891-92.     pp.  559-586 

Contams  important  observations  oa  the  first  appearance  of  the 
mouth,  and  its  relation  to  the  pituitary  body 

Cvt'^r'V^'?^-     ^'"''"'  ""  Ent^icklun,s,eschichte  der 

YlZl  l^  (^f  ^^^« '■^^^^''«-)  Zoologische  Jahrbucher.  Abth. 
I.  Anat.  VI.     1892.     pp.  161-444. 

J.T  "^Tf  I  ^^-  ^'^'^  '^'"  '''''^''"'  Neuroporus  und  die 
Pylogenetrsche  Function  des  Canalis  Neurentericus  der  Wirbel 
there.     Zool.  Anz.  VII.     1884.     pp.  683-687. 

WiLLEY,  A      Studies  on  the  Protochordata,I.-m.     Quarterly 
Jour.  Micro.  Sc.  XXXIV.  -XXXV.     1893.  Viuarreny 

Contain  further  bibliographical  references. 


128 


129 


130 


131 


132 


^33 


INDEX. 


Acipenser  sturio,  102,  129,  287. 

AcfdKia,  17,  46. 

Agassiz,  a.,  250,  251,  256. 

Allman,  262. 

Atnmocwtes,  163-170,  173,  178,  182,  186, 

282. 
Anprews,  39,  41. 
Annelid  tlieory,  5,   79,  82,  97,  176,  282, 

290,  293. 
Annelids,  excretory  system  of,  78-82,  99. 
giant  fibres  of,  97,  103. 
nervous  system  of,  95-97. 
segmentation  of,  4. 
vascular  system  of,  55. 
Antedon  rosacea,  256,  268-269,  271. 
Anus,  14,  25,  118,  131,  187. 
Aorta,  dorsal,  49,  50,  53. 
Aperture,  buccal,  182. 

cloacal,  182,  183,  210. 
Appendicularia,  180,  236-239,  241,  277. 
Archenteron,  no. 
Artery,  branchial,  47,  50,  98,  139. 

genital,  98. 
Ascidians,  pelagic,  181,  236. 

sessile,  181. 
Asterias  vulgaris,  254,  270. 
Asterina  gibbosa,  270,  271. 
Asymmetron  lucayanum,  40,  41. 
Asymmetry,  155-162,  177. 
Atrioporc,  14,  77,  105. 
Atrium  (see  also  Cavity,  peribranchial), 
14,  22,  186,  195. 
development  of,  75-78,  210-212. 
posl-atnoporal  extension  of,  25. 
Audition,  44. 

AL'DOUIN,  197. 

AtiricuLiria,  251-253,  256,  268. 
Axis  (see  Relations,  axial). 
Aykrs,  18,  173. 

Balancers,  42. 

Balanoglosstis,  29,  43,  98,   128,  221,  222, 

231,  242-253,   259,   261,   264,   26s,' 

274,  276. 

31 


Balanoglossus,  nervous  system   of,   244- 
246. 
h'owalevskii,  248,  250. 
Kupfferi,  248,  253. 
Balfour,  5,  38.  79,  175,  190,  203,  273, 

283,  292. 
Band,  adoral  ciliated,  250. 

circumoral  ciliated,  251,  256. 
longitudinal  ciliated,  251, 
post-oral  (circular)  ciliated,  251,  256. 
Bands,  mesodermic,  120,  217,  218. 

peripharyngeal,  34,  140,  145,  168-169, 
179.  185,  195,  226. 
Bars,  branchial  (see  Gill-bars). 
Bateson,  98,  221,   244,  245,   250,  259, 

263,  291. 
natrachtis  tau,  281. 
Bdellostoma,  173,  285. 
Beard,  208,  281,  292. 
Beddard,  81. 
van  Beneden,  187,  191,  197,  200,  224, 

291. 
Benham,  33,  42. 
Bert,  174. 
Bipinnaria,  251. 
Blastocoel,  108,  254,  255. 
Blastomeres,  107. 
Blastopore,  no,  112,  197. 
Blastula,  io8,  197. 
Blood-sinuses,  191,  192. 
Blood-vessels,  contractile,  47,  98. 

origin  of,  122. 
Bodies,  polar,  106. 
Body,  pineal,  207. 

pitituary  (see  Hypophysis). 
Body-cavity  (see  also  Coelom),  217,  220- 
222,  247. 
prn^'onil,  128,  218. 
Bojanus,  organ  of,  194. 
Botryllus,  181,  240. 
Boulenger,  14. 
Bourne,  A.  G.,  81. 

BovERi,  42,  48,  6c,  98,  99,  100,  151,  177. 
Brjchioliir  ill,  270. 


312 


INDEX. 


^'f 


Brain,  92,  101. 
Branchiomery,  65,  132. 
Dranchiostoma  cultcUutn,  40. 

lubricum,  8. 
Breeding-season,  105. 
Broo(i-poucli,  215, 
Brooks,  254,  277,  289. 
Bulbils,  vascular,  48. 
burckhardt,  284. 
Bury,  H.,  269. 

Caldweli,,  291. 

Canal,  alimentary,  24,  iii,  187,  196,  214, 
23s,  249,  264. 

neurenteric,  114,  118,  199,  202,  275. 
Capillaries,  49,  98, 
Capitdlida,  81. 
Cartilages,  buccal,  18, 147. 

labial,  18. 
Caulus,  266. 
Cavity,  opercular,  22. 

peribranchial  (see  also  Atrium),  22, 
183,  186,  19s,  209. 

peritoneal,  22. 
Cells,  epithelio-muscular,  191. 
Cellulose,  182. 
Cenogenesis,  177. 
Cephalisation,  75,  89. 
Cephalochorda,  13. 
Cephalodiscus,  261-267,  280,  289. 
ClKBtognatha,  278. 

Ciona  intestinalis,  203,  210,  215,  222,  224, 
226,   229,  230-235,  240,  271,   288, 
292,  293. 
Cirri,  buccal,  12,  20,  145. 
Cladoseldchidce,  44. 
CLAIM',  Cornelia,  281, 
Clavelina,  181,  185,   187,   200,  215,  225, 

241,  288. 
Cleavage,  107,  197. 

polymorphic,  108. 
Cceca,  intestinal,  249,  261. 
Civciliani,  67. 
Coecum,  hepatic,  24,  236. 
Couloni,  22,  26,  31,  33,  III,  121,  122,  220- 
222,  247-248,  265,  266. 

perigonadial,  153,  177. 
CcEnceciuni,263. 
Collar-pores,  98,  248,  265. 
Collar-region,  242,  264. 
Collector,  45,  165. 
Commissure,  circumocsophageal,  96,  273, 

280. 
Compression,  bilateral,  15,  43,  115. 


Contraction,  peristaltic,  98,  192. 

Cordon  ganglionnaire  visc6ral,  224. 

Cos lA,  7,  10. 

Craniota,  17. 

Criuoidea,  268. 

Cross  bars,  28. 

Cl'NNlNCHAM,  J.  T.,  80. 

Cutis,  38,  41,  122. 

CUVIER.  3. 

Cyclostomata,  8,  10,  45,  208. 
Cyclostome,  46. 
Cynthia  papulosa,  200. 

Davidokf,  200. 

Dean,  B.,  44.  • 

Degeneration,  5. 

Development,  abbreviated,  214,  215,  239, 

adolescent  period  of,  149,  150. 

direct,  250. 

duration  of  larval,  149,  169,  203,  215. 

embryonic,  114,  201. 

larval,  117, 130. 

latent,  145,  ijo, 

p.ecocious,  161,  212. 
Differentiation,  sexual,  154. 
Dissepiments  (see  Septa). 
Distaplia  magttilarva,  206,  225,  288. 
Distribution,  11,  40-41. 
Diverticula,  anterior  intestinal  (see  also 

Head-cavities),  115. 
DOHRN,  s,  30, 167, 173, 176, 178, 179,  280, 

281, 282. 
Duct,  mesonephric,  66. 

pronephric,  69, 78,  99. 
Dura  mater,  87. 

Echinodcrms,  250-256,  267-271,  291. 
Ectoderm,  24,  78. 

ciliated,  112,  113,  130,  175,  243,  257. 

definitive,  iii. 

primitive,  110. 
EISIG,  45,  81,  94,  103,  293. 
Embryo,  ciliated,  113,  214. 

ventral  curvature  of  Ascidian,  201. 
Emery,  67. 
Endoderm,  definitive,  iii. 

primitive,  no. 
Endostyle,  9,  24,31,  39,  130,  138,  149, 150, 

167,  177,  185,  195  227,  229,  250. 
Enterocoel,  252,  254,  255. 
Entcropneusta,  242. 
Epicoile,  41. 
Epithelium,  atrial,  33,59,  100,  209. 

coelomic,  33,  122,  220-222, 


l^iMM 


H:,,?M-. 


INDEX. 


ji3 


Equilibration,  44,  205. 
Equilibrium,  10  43. 
Erlanh;ek,  220. 
Evolution,  parallel,  80,  247,  290. 
Eye  of  Ascidian  tadpole,  102,  206, 
Eye,  median,  18,  102,  130. 

myelonic,  207. 

pineal,  207-209. 
Eyes,  paired,  102. 

Fascia,  36,  123. 
Fklix,  99. 

Fertilisation,  106,  188. 
Fibres,  giant,  92-94,  103. 
Miillurian,  94. 
of  Mauthner,  94, 
supporting,  89. 
FlKLl),  G.  W.,  254. 
Fieras/er,  67. 
Fin,  definitive  caudal,  131, 
provisional  caudal,  115. 
Fin-rays,  15. 
Fins,  15,  44. 

lateral,  38,  42. 
Fixation,  organ  of,  222,  229,  271,  280. 
Flemming,  99. 
Flexure,  cranial,  92,  279. 
FOL,  239. 
Folds,  medullary,  199. 

metapleural,  15, 38, 42, 43, 76, 132, 176. 
Follicle,  105. 
Food,  9,  39,  185,  249. 
Fowler,  G.  H.,  262,  ?.66,  267. 
Froriep,  175. 

Function,  change  of,  176,  280. 
Funnels,  atrio-coelomic,  58,  98. 
brown  (same  as  preceding), 
coelomic    (see   also    Xephrostomes), 
62. 

FUSARI,  87,  163. 

Fusari,  plexus  of,  87, 178. 

FURliRIiNtJER,  99. 

Ganglia,  peripheral,  85,  88. 

spinal,  84,  103. 
Ganglion,  Ascidian,  188,  224,  225. 

cerebral,  96,  270,  272-274. 
Ganglion-cells,  89,  91. 

bipolar,  95. 

giant,  92. 

multipolar,  92. 
Garstang,  240,  250. 
Gastrula,  no,  197, 

significance  of,  iii. 


Gastrulation,  log. 
GECEMiAUR,  249,  273. 
Germ-layers,  primitive,  no,  114. 
Gill-bars,  28,  32-34. 

blood-vessels  of,  48-49. 
Gill-pouches,  165, 166. 
Gill-slit,  first,  117,  118,  132,  141,  166,  170- 

172. 
Gill-slits  (see  also  Stigmata),  17,  27,  loo, 
130-132, 135-138, 139, 148-149, 160, 
173-174. 19s.  229,  234,  243,  244, 264, 
289. 
asymmetry  of,  157-158. 
atrophy  of,  140,  143,  149. 
Gland,  club-shaped,  116,   117,   134,  138, 
141,  170-172,  176. 
pyloric,  236. 
subneural,  188-191,  225. 
thyroid,  169-170. 
thymus,  29,  30. 
Glands,  fixing,  204. 
Glomerulus,  64,  65,  69,  100. 
Gnathostome,  46. 
Gobius  capita,  282. 
Goodsir,  8. 
i>E  Graak,  208. 

Groove  of  Hatschek,  21,  51,  135. 
Groove,  epibranchial,  226. 
hyperbranchial,  34,  39, 195. 
hyperpharyngeal  (same  as  prer  ding) . 
hypobranchial  (see  also  Encljstyle), 

9,  167. 
medullary,  112,  198. 
pericoronal  (see  Bands,  peripharyn- 
geal), 
peripharyngeal  (see  Bands,  peripha- 
ryngeal). 
Gut,  post-anal,  203, 

HAECKEL,  S,  46,  III,  177. 

Hancock,  190. 

Harmer,  263,  289. 

van  Hasselt,  193. 

Hatschek,  41, 91, 102, 103,  104, 112,  115, 

118,174, 175.  292. 
Hatschek's  nephridium,  172. 
Head-cavities  of  Ammocoetes,  129. 

of  Amphioxus,  126-128. 

pni-mandibular,  128,  175,  279-280. 

of  Sagitta,  277. 
Heart,  46,  51-53,  191,  192. 

recurrent  action  of,  193. 
Heider  (see  KoRSCHELTand  HElDER). 
Heptanchus,  173. 


314 


INDEX. 


Hkkdman,  183,  277,  293. 
Hermaphrof''te,  187,  196. 
Hexanchus,  173. 
HjOR  r,  225,  293. 

HUCHSTKI  TKK,  54. 

Hood,  nerve-plexus  of  oral,  84,  178. 

oral,  12,  147,  150,  178. 
HUHRIXHT,  258,  259,  260,  287,  291. 

HlXI.F.V,  20,  22,  41,  III. 

Hypophysis,  160,  165,  178,  190,  191, 
225,  283-288,  290,  292. 


I9S. 


Ichthyophis  ^'liifinosus,  67. 
Infundibulum,  102,  283,  285. 
Insects,  compared  with  Vertebrates,  2-4. 
Involutions,  atrial,  209,  241, 

JULIN,  187,  190,  197,  200,  224,  225,  226, 
292. 

Kastschknko,  175. 

Kidney,  65. 

Klinckowstrom,  207. 

Kolliker's  olfactory  pit,  19. 

Koi'l'KN,  103. 

KORSCHEI.T  and  HEIDER,  178. 

KOWALEVSKY,  4,  I04,   H4,  I74,  I96,  2l6, 

240. 
Krohn,  197,  250, 

KUPFFER,  101,  102,  128,  129,  175,283,287. 

Lamella,  post-oral,  264. 

Lamina,  dorsal,  183,  185,  195,  226. 

terminalis,  284. 
Lamprey  (see  Petromyzon). 
Lanc,  291. 

Lancerhans,  21,  56,  98,  loi,  154. 
Lanice  conchilcj^a,  80. 
LANKE.STER,   38,  4I,  58,  62,  98,  III,  237, 

262,  266. 
Lkuckart,  IOO. 
Levdu;,  4. 

Liganientum  denticulatvm,  25,63,  164. 
/.I  III  ax  lanceolatus,  7. 
Line,  lateral,  21,  42-45, 
Liver,  24. 

Lobe,   i)r;i'oral,  218,   222,  228,  229,  254, 
267-280,  290,  292. 

procL'phalic,  272. 
Lobus  olfactorius  impar,  102,  283,  284. 
Lc    imotion,  caudal,  103,  203. 

ciliary,  121. 

muscular,  lai. 
Loimia  iiit\iiisa,  8o, 


Lumbricus,  79,  272. 
LwoKK,  175. 
Lymph-spaces,  15,  51. 

MacBriue,  271. 
Mantle,  cellulose,  183. 

muscular,  183. 
NLVRSHALL,  MlLNES,  177. 
Maturation,  106. 
Mauthner,  fibres  of,  94. 
Mayer,  Paul,  99, 100. 
Medulla  oblongata,  91. 
Membrane,  intercoelic,  152. 

vitelline,  105. 
Aferlucius,  67. 

Mesenchyme,  201,  217,  220-222,  261. 
Mesoderm,   iii,  114,  120,  122,  199-201, 

221. 
Mesonephros,  66. 

Metamerism,  64,  132,  196,  246-247,  291. 
Metamorphosis,  136, 150,215, 223, 250,  256. 
Metanephros,  66, 
Metcalf,  293. 
Metschnikoff,  251. 
Meyer,  Eduard,  80. 

MlLNE-EmVARDS,  197. 

Ml  NOT,  155. 

M'iNTOSH,  263. 

Molgiila,  194. 

Molgula  manhattensis,  210,  232,  240. 

Morgan.  T.  H.,  232,  245,  247,  253,  256, 

274. 
Mouth,  19,  117,  131,  143-144,    146,  150, 

176,  178,  229,  276,  280-282. 
asymmetry  of,  157-160. 
MiJLLER,  Fritz,  250. 
MiJLLER,  J.,  8,  18,  50,  i;6,  59,  250. 
MijLi.ER,  W.,  102,  167. 
Muscles,   34-37,   86,   122,  195,  203,  222, 

235. 
Muscle-fibres,  origin  of,  121. 
Musculature  (see  Mus:les). 
Myocoel,  121. 
Myotomes,  13,  150. 
Myxine,  gill-slits  of,  171. 

hypophysis  of,  285. 

pronephric  duct  of,  100. 

Nansen,  103. 

Xassonoff,  190. 

iXemertines,  249,  256-261,  272,  273. 

lateral  nerves  of,  259. 

medullary  nerve  of,  259,  260. 
Nephridium,  62,  79,  99,  261. 


INDEX. 


315 


Nephrostomes,  65,  69,  72. 
Nerve-cord,  ventral,  96,  259,  273,  289. 
Nerves,  cranial,  85. 
motor,  86,  100. 
R.  branchiaiis  vagi,  163,  164. 
Rr.  cutanei  ventrales,  44. 
R.  reeurrens  frigcmini  et  facialis,  45. 
R.  cutaneus  quinti  (same  as  preced- 
ing) ■ 
R.  lateralis  trigemini  (same  as  pre- 
ceding). 
R.  dorsal  is,  85,  103. 
R.  lateralis  vagi,  45,  259. 
R.  ventralis,  85,  103. 
R.  visceralis,  86. 
sensory,  86. 
spinal,  83. 
Nerve-tube  (see  Tube,  medullary). 
Nervous  system,  origin  of  central,  m 

119.  198. 
Neuropore,    19,   90,    115,   jgo,    199,  202, 

223,  225,  283,  285,  287,  292. 
Norman,  Canon,  262. 
Notidanidce,  173. 

Notochord,  8,  13,  m,  115,  124-126,  158, 
161-162,  199,  216,  222,  244,  266^ 
286,  287,  290. 


Ontogeny,  177. 
Operculum,  264. 
Organs,  renal,  55,  194, 

reproductive      (see     also     Pouches, 

gonadic),    122,   151-155,   187-188! 

246,  266. 
Otocyst,  205. 
Otolith,  10,  205,  224. 
Oviduct,  187. 
Ovum,  105. 
owsjannikow,  100. 

Pagenstf.cher,  100. 
Palingenesis,  177. 
Pai.las,  7. 

Pallid  iia  vivipara,  220. 
Papilla-,  adhesive,  204. 

renal,  56-57,  59. 
Pericardium,  191,  218. 
Peric/icrta,  81. 

Petiomyzon,  93,  163,  169,  286. 
P/ialliisia,  203,  232,  292. 
Pharynx,  27,  183. 
Phylogeny,  177. 

Pigment,  18,  26,  33,  102,  130,  131.  134, 
304. 


Pigment-cells,  135. 
PtUdtum,  272. 
,  ^'♦'  o'fiictory,  19,  90,  145,  160.  165,  195 

283,  285,  292. 
I       pra'oral,  51,  128,  135,  144,  148,  267. 
Plate,  apical,  255-256,  269,  270,  272-274, 
292. 
medullary,  113,  115,  ns,  198. 
Plates,  skeletal  (endostylar),  32. 
Platt,  Julia,  175. 
PUuronectidcB,  3,  40,  162,  178. 
Plexus,  branchial,  163,  164,  165. 
Pliiteus,  268,  270. 
Pole-cells,  mesoblastic,  175. 
Pollard,  h.  B.,  282. 
Pontobdella,  8r. 
Porus  branchiaiis,  23. 
Pouches,  archenteric,  114,  115,  120,  247, 
248. 
gonadic,  13,  25,  40,  153-154. 
myocoelomic,  122. 
Pouch  ET,  82. 
Prist  hints,  99. 

Proboscis,  221,  242,  247,  257,  264. 

Proboscis-cavity,  247. 

Proboscis-pore,  128,  248,  253,  264. 

Proboscis-sheath,  258. 

Products,  genital,  174. 

Pronephros,  6(.<  -75,  78. 

blood-vessels  of,  63,  69,  74,  100. 
development  of,  69,  78. 
Prostomium,  272. 
Protopterus,  14. 
Pyrosoma,  181,  236,  241. 

Quatrefages,  88,  174. 

Rakl,  175. 

RAiiL-RUCKHARn,  284. 
Riiderorgan,  21,  148. 
Rathke,  8. 
Recessus  opticus,  102. 
Rectus  abdominis,  35. 
Relations,  axial,  226-229. 
Retzius,  82,  100,  103. 
Rhabdopleura,  261,  262,  266,  267. 
Ridge,  epibranchial,  226. 
Ridges,  subatrial,  76. 
RiTTER,  250. 
Rods,  skeletal,.  28. 
ROHDE,  100,  loi,  103. 
RonoN,  82,  86,  163,  165. 
RoLPii,  23,  41.  56,  86,  98. 

ROCKKRT,  60,  99,  100,  154. 


"fc/ 


316 


INDEX. 


Sac,  branchial  (sc  aL?  Pharynx),  183, 

195,  227. 
Sagitta,  13,  277-278. 
Saint-Hii.aike,  I,  279. 

principles  of,  2,  279. 
Sai.knsky,  206. 
Salpa,  180,  182,  193,  236,  241. 
Sarcolemma,  36. 
Sars,  G.  O.,  262. 
Savic.ny,  190. 
Schizoccel,  175. 
Schmidt,  Karl,  182. 
Schneider,  Anton,  35,  38,  98,  loo,  178. 
Sclerotome,  123,  175,  221. 
Sedgwick,  Adam,  112,  289,  291. 
Seklu'.er,  239-240,  269,  277. 
Segmentation  (see  Cleavage). 
Segmentation-cavity,  108. 
Semon,  67. 

Semper,  5,  79,  99,  176. 
Sense-cells,  20,  21. 
Sense-organ  of  prneoral  pit  (see  Groove 

of  Hatschek). 
Septa,  13,  37,  122. 
Sheath,  notochordal,  38,  123. 
Sheldon,  Lilian,  293. 
Shield,  buccal,  263. 
Skeleton,  axial,  13. 
Snout,  115,  218. 
Somites,  mesodermic,  115,  121. 
Spawning,  105. 
Species  of  Amphioxus,  41. 
Spee,  Graf,  99. 

Spencer,  Baldwin,  207,  208,  209. 
Spengel,  38,  41,  248, 
Spermatozoa,  105. 
Spinal  cord,  83,  222. 

central  canal  of,  89,  289. 
Spiracle,  173. 
Spiraculum,  23. 
SplanchnoccL'l,  122. 
Stage,  critical,  149,  174. 
Stannius,  45. 
Stieda,  100. 
Stigmata,  183,  195,  196,  227. 

formation  of,  229-235. 
StomodcEum,  165,  209. 
Sympathetic  system,  35,  86. 
Synapticula  (see  Cross-bars). 

Table,  showing  order  of  development  of 
Ascidian  and  Amphioxus,  213. 


Tadpole,  Batrachian,  14. 

Tail  of  Ascidian  tadpole,  201-204,  312^ 

222. 
TeUosteatis,  45,  281. 
Tentacles,  velar,  20,  195. 
Test,  182,  240. 
Thacher,  38. 
Thymus,  29. 
Tissue,  connective,  37,  41,  123. 

mesenchymatous,  221. 
Tongue-bars,  28,  140,  142,  148,  231, 
Tormria,  250-253,  255-256,  270,  274. 
Trochophore,  256,  272. 
Tube,  medullary,  114,  120,  198,  274. 

neuro-hypophysial,  225. 
Tubercle,  dorsal,  189,  225. 
Tuberculum  posterius,  102. 
Tubules,  excretory,  59-65,  72,  100, 122. 

mesonephric,  70,  177. 

pronephric,  67,  70,  78, 100. 

uriniferous,  65. 
Tunic  (see  Test). 

Ureter,  66. 
Urmund,  no. 
Ussow,  190. 

Vacuolisation   of  notochord,   125,   216, 

240,  244. 
Vas  deferens,  187. 
Vein,  cardinal,  54, 

caudal,  54. 

hepatic,  49,  98. 

portal,  53,  98. 

sub-intestinal,  49,  53-55. 
Velum,  20,  50,  150,  178. 
Vesicle,  cerebral,  90,  100,  204,  223,  224,. 
226. 

Water-pore,  253,  254. 

Weiss,  F.  E.,  57,  59. 

VAN  WlJHE,  39,  50,  51,  88,99,  "8,  163, 

164,  165,  178,  289. 
Wilder,  Burt  G.,  14. 
Wilson,  E.  B.,  108,  174, 175,  292. 
Woodward,  A.  S.,  44. 

Yarrell,  8. 

ZiEGLER,  H.  E.,  175. 
Zoarces,  67. 


